A key misconception is this:
When we stop putting carbon in the air, Earth’s surface temperature stops rising.‡
Instead, warming only stops accelerating then. It only stops rising when the feedbacks we have unleashed, mostly changes in reflection (albedo) and water vapor, also natural emissions (permafrost etc.), have played out. That’s when Earth’s surface warms enough that the energy leaving the top of the atmosphere = the energy absorbed at the surface. These albedo and other feedbacks are large. Already responsible for 1/3 of observed warming, by 2035 they will likely exceed the direct effects of our greenhouse gas emissions. Even at equilibrium, temperatures will be quite a bit higher than today's, since we did not go to net-zero immediately.
Due to these feedbacks (clouds, water vapor, sulfates, snow, sea ice, permafrost, land ice, black carbon from forest fires, etc.), and to our legacy emissions and our near-future emissions, we cannot hold global warming to 1.5°C. Only with extraordinary efforts, can we hold it to 2°C. Current efforts are ordinary. Global land surface warming since immediately pre-industrial (1880) is already 1.86°C (NASA, 5-year mean, but 2.08°C 1-year mean), while sea surface warming is only 0.77°C (0.87°C 1-year mean). Land and sea combined yields 1.05°C (NASA 5-year mean, but 1.23° comparing 1-year means) or 1.04° NOAA (5-year mean, 1895-2023, 1.15°C 1-year mean).
The last several times that Earth had this much carbon dioxide (CO2) in the air (14-15 million years ago), Kansas was hotter than Las Vegas now and Florida was mostly underwater. (See 2 graphs below: Vostok Ice Core Data & Lessons for Our Future from Long Ago.) The feedbacks responsible for these changes milllions of years ago are mostly the same ones that turned the effect of small variations in Earth's orbit and tilt into several ice-age cycles, with temperatre swings (more slowly) that were several times as big as we have observed in the past century.
To hold global warming to 2°C, we humans needed to stop putting carbon in the air around 1960. To hold global warming to the current 1.2°C, we needed to stop putting carbon in the air around 1912.
To hold warming to 2°C, we need to not only (1) end our emissions, the sooner the better, but (2) remove the legacy CO2 (some 1.8-2.7 trillion tons) that we’ve already put in the air, plus our emissions between now and 2050, AND (3) cool Earth, by increasing Earth's reflectivity (cloud changes, etc.), especially in the Arctic*, and by increasing outbound infrared radiation at the 2 infrared wavelength ranges unaffected by greenhouse gases (see graph at top of Tons, PPM $, SRM page). CO2 removal is too slow in scaling up to avoid the need to cool Earth.
* Arctic warming causes the north polar jet stream to slow down and wander more. The bigger meanders bring longer droughts, floods, heat waves and cold spells.
I worry about amplifying (positive) feedbacks that increase temperatures, without changing (directly) greenhouse gas levels in the air. Feedbacks overlap with tipping points. Loss of northern (Arctic) sea ice and its reflective power (see diagram above) is the feedback most often talked about, and one of the soonest (years to decades) tipping points. Less ice yields more heating yields still less ice, yields still more heating. Snow cover loss is similar. Southern sea ice area has plummeted since 2020, for an even larger feedback. A somewhat bigger, and sooner, feedback, is well under way since 1979. It comes from loss of cooling sulfates in the atmosphere, as we stop burning or scrub coal. Some major feedbacks, like more water vapor (#1 greenhouse gas) in the air as it warms, are not tipping points. The other key near-term amplifying feedback is cloud cover changes. Land ice loss (especially near the poles - thick ice) feedbacks come mostly later and more slowly (centuries to many millennia). Cloud cover changes may accelerate, in perhaps a tipping point, as the middle troposphere become "too" warm. See the Clouds section on the Heat page. Other tipping points that we may have passed already include initiating coral extinction, and losses of eastern Amazon rainforest, Greenland ice sheet, West Antarctic ice sheet, nothern sea ice, and southern sea ice. But if we reduce atmospheric temperatures soon enough and fast enough, we may be still able to reverse what we have initiated. That's a very tall order.
Natural Climate Change
Earth's climate has been changing for hundreds of millions of years (MY).
It will continue to do so. Climate change is nothing new.
Earth has had this much CO2 in the air before - most recently 14 million years ago. With current CO2 levels, we will nearly repeat the conditions on Earth back then. Back then, Earth’s surface was 5-8°C hotter than in 1880 and seas averaged 100 feet higher.
Current warming is 7-30 times as fast as the previous fastest warming (over 1,000+ years) in the geological record. Based on this record, we can expect 3-4 times as much warming as we have experienced so far (since 1750), from current greenhouse gas levels. Most of this lag effect will come from (1) changing how much sunlight Earth reflects (with much less snow and sea ice, no more sulfates (from burning coal and diesel), and less* cloud cover); plus (2) further carbon emissions from thawing permafrost (plus methane hydrates, warming upper ocean, and warming soils); and (3) more water vapor (the #1 greenhouse gas) in warmer air. A bit will come from (4) slowly heating up the the entire ocean deeps until Earth’s energy in = energy out. (Oceans already gain almost as much energy (heat) every 2-3 years as all the energy humans have ever used.)
The last 2 times Earth had about this much CO2 in the air (about 4 and 14 million years ago), global surface temperatures averaged 7°F (4 Mya) and 10°F (14 Mya) warmer than now and seas were 65-130 feet higher. In other words, Kansas then was hot as Las Vegas now and Florida was mostly under water. Similar around the world. With current CO2 levels, we will nearly repeat the conditions on Earth back then.
The difference between the low and high ends of the ranges - in the 3rd, 4th and 5th diagrams below - shows how conditions then changed over a few hundred thousand years, a time span like 2 to 4 of our “recent” ice ages. We can expect today’s 421 ppm of CO2 to have that effect again.
Lag effects of today’s CO2 & methane (CH4), much of it from compounding albedo changes when most snow and much of Earth's ice vanishes, are very large. They will make summer highs in Kansas, Oklahoma, and Texas - even South Carolina, Georgia and Idaho - hotter than those in Las Vegas now - by 2100 if the warming pace over 1995-2015 continues.
For at least half the last 600 MY, Earth's surface has been 15-30°F warmer than now, with CO2 levels at least 5 times today's, much higher till 350 MY ago.
Starting ~ 375 MY ago, CO2 levels fell steeply, after plants colonized land. Glaciers were widespread ~300 MY ago.
Oxygen levels generally fell when CO2 levels rose, and vice versa. That’s also true, but subtly, over the past century.
In part, higher CO2 levels compensated for a dimmer sun in the distant past. Our sun brightens slowly as it ages, warming Earth about 2°C more per 100 million years. Clouds modify that.
Our sun also brightens, then dims, by ~ (about) 0.1% (from minimum to maximum), during "11-year" sunspot cycles (9-13 years, usually 10 to 12). More dark sunspots are outshone by more bright faculae around them, for a brighter sun, especially in ultraviolet light. 2008 saw the fewest sunspots in 111 years, so it’s no surprise that solar radiation was the lowest in the 40-year satellite era. That means that changes in solar output generally had a slight cooling effect on Earth over the last 40 years. Variation during longer (and more variable in length) sunspot cycles is subtler. Solar changes do influence climate, but their effect has been overwhelmed by the much larger effects of human emissions. The sun has cooled slightly since 1979, so humans emissions (with their feedbacks) account for over 100% of the warming observed since then.
In the satellite era, Earth warmed as the sun brightened, and cooled as it dimmed, mostly - until 2002. During 2002-2008 and 2014-16, Earth’s surface temperature rose while solar output fell. Again, solar variability’s modest influence on climate has been overwhelmed by the human effect.
(Almost all graphs on this website are JPGs. Graphs that Dr. Fry created are generally available in many of his slide shows, as editable PowerPoint slides, some with underlying Excel spreadsheets. Dr. Fry’s graphs feature Arial Bold font, larger than in most other people’s graphs, with gridlines and a few somewhat thick data lines of various colors. They include the solar radiation graph above / left and the graphs below for paleoclimate data analysis, recent CO2 levels, recent global temperatures, and sulfate-temperature interactions; in fact all but 4 graphs below.)
CO2 levels have varied a lot over the eons (see above for 570 million years). Very long ago (~250 MY, etc.), vast lava eruptions lasting a million years or so, such as the ones that created the Siberian Traps and India's Deccan Traps, and even the Columbia River Basalts later, added lots of CO2 to the air: multiples of what we have now. 20th century volcanic eruptions added far more modest amounts: roughly 1% of current human CO2 emissions.
When continents collided, mountain ranges rose. As moist air moved over them, they caught more rain and snow. This speeded up rock weathering processes, which annually remove ~3% as much CO2 from the air as humans now put in. In weathering, CO2 dissolved in rainwater combines chemically with minerals (mostly calcium silicates) in rock surfaces, which wash away to become carbonate sediments. This has been so for 100s of MY.
During the PETM's temperature spike 55 MY ago, global temperatures rose by 0.025 to 0.06°C per century. To compare, surface warming over the past century was 1.2°C. This has speeded up. Over the past 20 years, the warming rate was 1.7°C per century. 2.2° per over 10 years.
Weathering in the Himalayas has driven CO2 levels down for some 50 MY, from some 3 times current levels (see 1st figure above). Ebbing CO2 enabled glaciation in Antarctica starting 34 MY ago. As CO2 ebbed further, Greenland glaciation began 18 MY ago and hit its stride 8 MY ago. Finally, as CO2 levels fell further, widespread glaciation began in Canada, Alaska, Scandinavia, most of Siberia and the northern US ~ 2.5 MY ago (punctuated by interglacials). Note how glaciers ebbed in Antarctica from 27 to 15 MY ago, as temperatures rebounded by 1 to 2°C for 14 MY.
Algae, plants, and seashells also removed CO2 from the air, to store in soil, water, and sediments, once life took hold. When life spread to land, the process speeded up (see above, 350 to 300 MY ago). As conditions permitted, these dead lifeforms transferred into coal, oil, gas, and limestone, which stored carbon underground. This has also been going on for hundreds of MY.
During ice ages over the past 2 MY, CO2 & CH4 levels were much lower than today's (~ 420 parts per million [ppm] of CO2 in the air, 1900 ppn of CH4).
The timing of these ice ages (once general CO2 levels dwindled enough - credit the Himalayas & an isolated continent at one pole), was driven by Milankovich cycles - small variations in Earth's tilt, the roundness of its orbit, & when it's closest to our sun (in northern [land hemisphere] summer, winter, or in between). The "beat", when these rhythms reinforce each other, comes about every 100,000 years.
Finer time resolution for these ice ages shows warming came 1st, followed "shortly" by more CO2 and CH4 in the air. Warming, from natural orbital factors, drove carbon out of permafrost (CO2 where it's dry, CH4 where it's wet), oceans, and soils. Warming speeds up decay by soil microbes.
Worldwide warming averaged .04°C per century from 20,000 to 10,000 years ago, but almost twice that at Vostok.
The relationship of CO2 (and CH4) to temperature about 4 and 14 million years ago is essentially the same as shown in the more recent Vostok ice cores. See graph below.
The green line connects the patterns relating temperature change to changes in CO2 and CH4, over the last few ice ages, to those the last times (4 & 14 Mya) we had about this much CO2 (and perhaps CH4) in the air. The purple line is just for the ice ages, since we don't know CH4 levels then. The story is consistent across millions of years. The equation including CH4, shown in green, has the best explanatory power (R2) for Vostok. The natural logarithm (LN) form adjusts for GHG molecules getting in each other’s way as they become more numerous, so that a smaller fraction of them actually absorb outbound infrared radiation over a given timespan.
With current CO2 & CH4 levels, the equation yields global warming of 8.6°C.
The purple line connects the patterns relating temperature change to changes in CO2 only. Neglecting CH4 provides a fit to the ice core data that is not quite as good. It implies just 5.3°C warming from today’s CO2 levels.
(Since 1750, CH4 levels have risen far faster than CO2 levels, 170% vs 48%, especially in the 19th and early 20th centuries, also in the 2010’s.)
What’s going on? Over a few decades, sea ice will vanish, as will sulfates from coal power plant smokestacks. Over many decades, snow cover keeps shrinking, arriving later and especially leaving earlier. Antarctic and Arctic sea ice may vanish at similar times, depending on future human emissions, carbon removal from air, and physical processes yet only partly understood. Antarctic ice began declining steeply 2 yesrs ago. All these changes make Earth darker, so it absorbs more heat.
More important, over the past 30 years, Earth’s cloud covered area has probably been decreasing by .06% per year. That makes Earth darker, so it absorbs more heat. This is actually a very large number, since Earth is large and half covered in clouds. The effect of the observed cloud cover decrease (in 2013 AMO report) is about half as much over 30 years as the warming effect to date from all greenhouse gases combined.
That’s assuming that the ratio of high-altitude clouds to low-altitude ones stayed roughly the same. However, since 2001, high altitude (net warming) clouds have increased. Since 1983, low altitude (cooling) clouds have decreased, while middle altitude clouds have increased. (See Heat page, Clouds section.) Finally, low clouds have been growing more opaque (Clouds section); this opaqueness cooling effect partly offsets the shrinking area and the growing high-to-low cloud altiude effects. The overall cloud effect, a fast feedback, is a multiplier; it currently adds an estimated ~20% to warming from other factors.
In addition, smaller albedo changes come over many decades to some millennia, as ice vanishes from almost all of Greenland and West Antarctica. (In recent years, ice loss has accelerated 12% per year in Greenland and in Antarctica.) Earth’s surface continues darkening. Thus, Earth absorbs still more more sunshine, heating up more. This is a amplifying ("positive") feedback loop. Most of the heat absorbed goes into the oceans, but a small fraction heats soil and rock. Even smaller fractions melt ice and heat the air. The smallest fraction (about 1%) heats the air, which is mostly what we pay attention to.
All these albedo changes will be multiplied by more water vapor in the air. Water is the #1 greenhouse gas. It amplifies warming from other causes: not only other greenhouse gases, but also albedo changes. Air holds 7% more water, at the same relative humidity, for each 1°C warming. The difference between relative humidity and absolute humidity is quite important: more water vapor in warmer air does not mean more clouds. The 7% more water vapor increases the heat effect (“radiative forcing”) by 1.5 Watts / square meter, about 2/3 as much as all other factors combined, or another 0.6-0.7°C water vapor feedback for 1°C warming.
This amplifies the direct effect of CO2, CH4 and other greenhouse gases - a lot.
But it also amplifies the effects of albedo changes.
5.3°C global warming is plenty to make Kansas as hot as Arizona.
8.6°C make Kansas hotter than the Sahara.
Earth has natural cycles that last for years. El Niño / La Niña is the most prominent of these.
Elevated CO2 levels have long been used in greenhouses to increase production of flowers and vegetables. This uses the "CO2 fertilization effect", boosting yields by 6-35%. However, plant growth is limited by many factors - temperatures too hot or cold (e.g., below freezing, or hot enough to denature key proteins), low light levels (night, etc.), CO2, water, nitrogen, phosphorus, potassium, acidity, soil depth, etc. When CO2 is a limiting factor, adding more CO2 helps plants grow, especially C3 plants - until the plants run low on another factor, most often nitrogen. This explains why experiments find that an initial (1-5 year) CO2 growth spurt fades. C3 plants include most food crops and weeds, but corn and sugar are C4 plants. See “Food” page, in “Summaries of Observations” and above “Future Impacts 2” for graphs of CO2 fertilization effects, most combined with temperature effects. However, while plants grow more carbonhydrates, they grow fewer proteins. See Food page for explanations.
Human-Accelerated Climate Change
Especially in the past century, humans have added lots of CO2 to the air - by burning coal, then oil, then also natural gas. Cutting down forests and farming practices (tilling, feedlots, flooded rice paddies, etc.) are other major factors.
These can, and do, speed up natural climate change.
The last times CO2 levels were almost this high, Earth’s surface temperature was about 7°F warmer. When they were slightly higher than now, more like 12°F hotter.
After a Medieval Optimum and 3 Little Ice Ages (see Heat page), land surfaces warmed from 1890 to 1942, plateaued to 1974, warmed steeply to 2004, little till 2012, then shot up again.
Sea surfaces warmed 44% as fast as land surfaces over 1880-2023. This includes 89% over 1963-83, 39% over 1983-2003, and 79% over 2003-23.
Q: If CO2 causes warming, why doesn't warming increase as smoothly as CO2?
A: Sulfates. See below.
Note that global land surface temperatures are ALREADY almost 2.0°C above 1880 (5-yr moving average), according to NASA (1.9°C says NOAA). They are more than 1.5°C above the 1750 level, a stretch target in the December 2015 Paris Accords. And they are not headed down.
. Earth's land surface has warmed about 1.4°C (2.5°F) since 1921. That includes 2.1°C (3.8°F) per century since 1971 and 2.4°C (3.7°F) per century since 2001. Warming since 1920 has been 70 times as fast as it was 11,000 to 7,000 year ago, or ~35 times as fast as from 18,000 to 11,000 years ago. And still faster in more recent years. Earth's surface has been warming far faster than before humans put lots of CO2 in the air.
90% of Earth's heat gain went to warm the oceans (including 80% to only 700 meters deep), while only 1% went to warm the air - which is mostly the warming we care about. The rest melted ice (2%), heated rock & soil (5%), and increased water vapor in the air.
The rate of ocean heat gain has accelerated, especially since 2005. Since 2005, oceans added heat 25 times as fast as humans now use energy (~150 x as fast as the US does).
Since 1969, ocean heat gain exceeds 10 x the energy humans have ever used: ~ 3,000 years of current US energy use. The tail is wagging the dog. (Humans are wagging Earth's oceans and air).
However, heat ≠ temperature. Heat = temperature x mass x heat capacity per mass. Since the oceans weigh 260 times as much as our air (& water has 4 x the heat capacity of air), air* has warmed much faster than oceans. This is somewhat so at the sea surface, but dramatically so for the deep ocean, which is warming slowly.
* The troposphere, where almost all of Earth’s air is located, is the part of the air that is warming. The stratosphere is cooling.
For heat to seep down from the sea surface to the deep ocean takes a long time. If CO2 levels in the air levelled off, the deep ocean would continue to warm slowly, until a new thermal stratification equilibrium is achieved. In 2011, it was estimated that such a new equilibrium would eventually warm the sea, and sea surface, another 0.6°C. It now takes roughly 10 centuries for the global thermohaline circulation, now driven by (diminishing) formation of deep water off Greenland and Antarctica, to make one circuit. This is a very rough estimate of the time it takes to warm another 0.6°C in response to TODAY’s greenhouse gas levels.
Air temperature WAS warming 50 times as fast as the [deep] ocean, but is NOW warming only 25 times as fast. Heat has been seeping faster into the deeper ocean. So, we saw a 2004-12 slowdown in warming AIR across the world.
Global warming has accelerated since 2005. The 2004-12 ‘hiatus’ (which ended) was confined to the air, NOT the ocean. That air “hiatus” has reversed, in spades. See the underlying sulfate data partly below and partly on the Heat and Tons, PPM, $, SRM pages.
The graph above shows much of why air has not warmed as smoothly as CO2 levels rose.
Moving averages include 2017, where appropriate.
Effects of sulfates are clear for major volcanoes, which put sulfates in the stratosphere for many months. (Smaller eruptions - too many to show - had much smaller effects, as most did not reach the stratosphere.)
The bigger picture (numbers 41-123 in bottom box of the Temperature / Sulfates slide, also near the bottom of the “Heat” webpage) is that, over 135 years, human sulfur emissions mostly rose, especially 1940-80, to peaks in 1973 and 1979. That masked much warming from CO2. But when sulfate levels fell (1930s, 1975-2005), warming was unmasked. Then temperatures rose steeply.
The graph below traces these factors, plus the two principal greenhouse gases, over the same time period. It shows that the close agreement between global surface temperatures and cumulative CO2 emissions. The historical explanatory power, 95.3%, however, is not as good as 98.3% for global land surface temperatures adding SO4 and CH4, above. Note that neither analysis includes the less important, but non-negligible, influences: black carbon, N2O, CFCs, O3, and solar changes.
Above, note the Mt. Pinautubo eruption in 1992 and El Chichón in 1982.
Now, switch tracks to rising temperatures in the USA.
Focus on summer highs.
One result of added CO2 has been warming in America. From 1975 thru 2015, daily summer highs in 26 places* around the US have increased, on average, by 3°C (5.4°F / century), a bit faster than Earth's land surface as a whole. From 1975 to 1995, the 26 barely warmed. (* Jointly, the 40-year urban heat island effects shrank (cooled) slightly for these 26 places. They were chosen to avoid places with rapid population growth, and thus increasing urban heat island effects, as US energy use did not increase faster than population.)
But since 1995, these 26 cities warmed 10°F / century. The summers of 2011 and 2012 were especially hot. Maybe it's just year-to-year variability. Maybe it it's more than that. But if warming continues as fast as it did from 1995 to 2015, by 2100 summers in Kansas, Oklahoma, Georgia and South Carolina would be hotter than Las Vegas ones now. That’s bad for crops. Summers would be still hotter in Texas.
Still, 26 is a small sample and 20 years is not so long. So, error bars on extrapolation are substantial.
With a larger sample of 330 places, the picture is very similar. See the Heat page for regional graphs and the Data page for individual city graphs and data. The results are similar, but the warming is a little faster (+5.8 and +10.5°F / century). Some of this modest (5.4 to 5.8 and 10.0 to 10.5) difference is probably an increase in the urban heat island effect.)
Warming varies a lot from one region to another. It was been fastest in drier areas: Rocky Mountain and West South Central states. Warming was also fast in South Atlantic states - except Florida. Warming was slowest in North Central states and Alaska.
Why was warming fastest in the Rockies and West South Central states? The #1 explanation: On land, sunlight absorbed evaporates water, if available; otherwise it mostly heats air. At sea, it also heats water below the surface. Preliminary research on US daily winter lows and daily summer lows shows only weak and ambiguous trends, lending further support to this hypothesis.
How long until summer highs average as hot as Las Vegas ones today, IF recent trends continue?
1995-2015 trends, especially in the faster warming areas, are truly sobering.
If these 20-year rates (which vary by place) should continue, today’s Las Vegas summer heat comes to Fresno in 2036 & Austin in 2037.
At the state average level, Nevada, Texas and Arizona summer heat would be as severe by 2083. Idaho, South Carolina, Kansas ("breadbasket of the world”), Oklahoma and Georgia would grow as hot shortly before 2100.
Humid areas - such as Georgia, South Carolina and Alabama - could become uninhabitable during summers late this century.
Slowing and even stopping such warming is utterly important.
Daily low temperatures, summer and winter, have not risen much or have even fallen (winter lows, 2nd half of 42 years). But summer highs have risen fast. Evaporation is higher then and soil moisture levels can be lower. See Heat page for more summary of summer and winter lows and the bottom of the Data page for details on them.
Oddly enough, since 1994, when (in any single year) Earth's land surface warmed faster than its trend, the US usually warmed slower than its trend, and vice-versa. (Correlation = -0.39.) Food for thought and investigation.
2012's summer heat could become the new normal within the lifetimes of most of us, by 2030 in several states.
In the longer term, unless carbon emission rates are cut sharply, by 2200 a few areas (e.g. Persian Gulf) could become too hot and humid for humans to survive in. They already are for a few hours per year, absent air conditinoing. Even Alabama, Georgia, and South Carolina sometimes in the summer. If CO2 levels reach 650 ppm and CH4 evels rise similarly, by 2300, large sections of Earth (e.g. most of Australia, India, the Middle East, the southern US, Mediterranean lands, and parts of Latin America and China) could become too hot and humid for humans to survive.
Since CO2 emissions are a major cause, to slow down heating or even stop it, we must do 3 things.
1. Stop burning money (use energy efficiently), whether or not CO2 is a problem.
2. Use low- & no-carbon energy (more wind and sun [now the cheapest energy in many areas]. Use nukes for at least a few decades. Use less coal (and oil). Long term, phase out coal now, gas soon, then maybe nukes. If we burn all fossil fuels in the ground, Earth would become ice-free and seas would be more than 200 feet higher. Most of Earth would be too hot for humans to survive. Ditto for most mammals.
3. Take carbon out of the air faster than we put it in. Pay ranchers & farmers to put it back in soils, etc. Use direct air capture, accelerate rock weathering, de-acidify the oceans, etc.
Warming has consequences. Perhaps the most important is faster evaporation, which helps bring intense droughts more often. (Between droughts, we get worsening floods.) Here is a projection from 1989, in 2 forms. Larger views appear on the Overviews page and in the middle of the slide show (available below).
Now, change from looking forward to looking backward: the world below left, the US below right.
Droughts HAVE, in fact, been increasing.
Observation up to 2002 matches projections made in 1989. Worldwide, severe droughts have been increasing.
Growing droughts are bad for our food supply.
World grain production per acre plateaued (bottom line in table at left) from 2008 to 2012, but rose 8% in 2013-14, due mostly to record US yields. We added more people and fed more animals for meat. In 2022, due to crop shortfalls in India, the USA, Europe and China, and war in the Ukraine, food prices rose.
Current agricultural models estimate that climate change will directly reduce production of corn and wheat (and with yet more warming, rice, and eventually soybeans too). These 4 supply 80% of human calories. Tropics suffer first, temperate zones later (with overlap). One model estimates a reduction up to 43% by 2100, another more. The IPCC now estimates 17%. With shrinking fresh water supplies, especially dwindling groundwater, further large declines loom, absent big improvements in water use efficiency. Pressure on food supplies is not likely to stop after 2100.
Less food feeds fewer people.
Reducing Earth's population to fit the future food supply will not be pretty.
Think Bosnia, Rwanda, Darfur, Somalia. Boko Haram, ISIS. Multiply.
However, many countries already face shinking populations: Japan, China, Korea, and many in Europe. Many others, including the US, Europe's other countries, and recenly India, now have fertility rates below replacement levels
Some aspects of climate change we know much better than others.
Perhaps the greatest uncertainties are for
1. how much and how fast we can move CO2 from ambient air to carbon sinks;
2. future carbon escape rates from permafrost and methane hydrates;
3. future human carbon emissions and the speed of that reduction;
4. future carbon escape rates from other soils and a warming ocean, plus changes in land biomass;
5. large scale changes in % cloud cover;
6. future effects on the food supply;
7. how fast snow and land ice melt;
8. future rates of sea level rise (see #7, Antarctic & Greenland melting);
9. how much future warming is unmasked, as we stop emitting sulfur from coal;
10. loss rate for sea ice;
11. effects at regional (e.g., Australia) and sub-regional (e.g. Britain) levels;
12. willingness to cool Earth while we reducing our forced heating of it.
Below is a set of projections for how the future will unfold, to 2400. They include many albedo (reflection) effects: reduced snow, sulfates, ice cover and cloud cover. They include large effects from more water vapor in the air as it warms. They include carbon emissions from permafrost. They include very rough estimates for carbon losses (jointly) from methane hydrates, currently unfrozen soils, de-gassing from warmer oceans, and reductions in land biomass. They also include modest effects for warming the deep ocean enough, so Earth’s outgoing radiation balances its incoming solar radiation.
The overall results are consistent with the warming equations derived from the Vostok ice cores (see Figure toward the top of this page, Lessons for Our Future from Long Ago) - over the CO2 concentration ranges for the past 15 million years. The CO2-only equation is ∆°C at Vostok = -107 + 19.1 * LN (ppm CO2). Using today’s very high methane (CH4) levels would make the picture appreciably worse than shown below.
However, the equation is a shorthand for physical processes. Warming increases as CO2 increases - both directly and from albedo and other feedbacks. As snow and ice become scarcer when Earth’s surface warms, those albedo feedbacks will produce less additional warming. When they disappear, only the direct effects of CO2 and other GHGs remain, plus their feedbacks from cloud changes and more water vapor in warmer air. Thus, the coefficient 19.1 shrinks, quite a lot, as snow and ice grow scarce and vanish. That effect is visible in the graphs below.
Only in scenarios featuring vast amounts of CO2 removal are future temperatures consistent with the continued existence of civilization. If human CO2 emissions do not peak soon and CO2 removal does not take place, Earth’s surace is likely to warm 9°C or more by 2400. This may not be consistent with the continued existence of the human species.
Permafrost emissions are based on MacDougall (see Overviews page). They correspond to DEP 6.0 in the Base Case, 4.5 in the case where fossil fuels are eliminated by 2050, and 2.6 in the case that also includes very large-scale carbon capture and sequestration - from ambient air. Higher DEP cases (than 2.6 emissions in Dr. Fry's Base Case) are used in Dr. Fry’s modeling above, to compensate for demise of cooling from sulfates and Arctic sea ice, plus cooling from fewer clouds observed in recent years (see Heat page, Clouds section), all of which were not part of MacDougall’s model. Systematic changes in cloud cover are subtle. See “Clouds” on the Heat webpage for details.
McDougall is conservative in several other ways, mostly as he states: 1) his estimate of the permafrost carbon pool is only 54% of more recent estimates, 2) he does not include methane hydrates (which recently emitted 30% as much carbon per year as permafrost), 3) he does not account for CH4 release from permafrost, 4) he does not account for thermokarst and water erosion of permafrost, 5) he does not account for permafrost warming by black soot from increasing near-Arctic forest fires, and 6) his temperature estimates for the permafrost region are at the low end of the range for the literature.
Future temperature changes will likely be dominated by loss of snow and ice, sulfates loss (in the short temr), permafrost carbon emissions (later), diminishing cloud cover, and increasing water vapor. Well before 2100, elimination of sulfates (a coal by-product that increases cloud cover) and polar sea ice play substantial roles. (See future temperature graph above from 2010 to 2070.) They both induce positive feedbacks from permafrost and clouds. Shrinking sea ice and snow cover play larger but similar roles, over decades to a few centuries. Looming above all is the water vapor warming amplification effect.
Future ice loss rates from Greenland and Antarctica are uncertain. But several new ice dynamics processes have been identified that were not in earlier models. All of them speed up ice loss projections. So, for example, DeConto and Pollard (2016) cut their estimated melt times for much of Antarctica from a few million years (2013) to a few hundred (2016). Sea level rise shown here to 2100 is half that projected by Hansen et al. (2016), but to 2400 is similar to DeConto and Pollard (2016).
The general temperature increase is very consistent with Snyder (2016, see Overviews page for summary). Also, based on more detailed paleoclimate data than used in the graphs above, Snyder foresees 5°C eventually from today’s CO2 levels and 9°C with 560 pm CO2 (double pre-industrial). But she does not envision the warming to happen as quickly as I do; short-run warming of about 3°C.
This all points to the utter importance of removing from the ambient air most of the carbon humans have put in it by burning fossil fuels. The sooner the better, much of it this century. It would be good to go carbon negative as a species before 2050. Civilization cannot survive the temperatures indicated if we fail to do so. The world food supply would be cut in half, or worse, at 5°C global warming. 9°C warming would be far worse. Past major extinctions have been driven by large climate changes due to copious CO2 emissions.
Some people are looking into ways to remove carbon from the air and store it in soils, rocks, trees, or oceans. Carbon removal is a major topic on the page "Carbon + & -". Others are looking at ways to block sunlight from reaching Earth's surface, a geo-engineering topic discussed on the “Tons, PPM, $" page. Blocking sunlight will not help with ocean acidification, but enough carbon removal will.
In 2013-14, China began CO2 cap & trade in 7 large emitting places. It started a nation-wide system in late 2017. As a result, China’s CO2 emissions fell for 3 years, before rising modestly again. And the rise in world CO2 emissions slowed. (See details on the page “Tons, PPM, $”.)
China is cutting coal use because (1) its air has been so terribly polluted,taking 5-6 years off a typical Chinese life, and (2) climate change threatens China’s well-being, with sea rise drought, and heat.
In 2020, the US emitted the least CO2 since 1992, including a 12% drop in the Trump era. In 2023, the Us is emitting even less CO2.
In many places, India’s air is even more polluted than China's. India has begun to follow in China’s footsteps. It is leveling off its coal use and canceled many proposed coal plants. As solar gets even cheaper, cooling water shortges lead to more thermal power plant (coal, gas, nuclear) shutdowns at inopportune times, and richer Indians do not want their lives cut short by 3-7 years. So, India is expanding its renewable electricity production exponentially and will end petroleum vehicle sales by 2030.
Meanwhile, on June 1, 2015, 6 major oil companies called for a worldwide carbon price. They already use internal “shadow” prices such as $40 or $34 per ton of CO2. In the run-up to the December 2015 Paris climate summit, 32 renowned economists, including 4 Nobel Prize winners, called for a worldwide carbon tax. On January 17, 2019, 1,000s of American most famous economists, including 97% of Nobel laureates and 100% of living former Fed Chairs, made the same call.
Again, we can all (1) use energy more efficiently, saving money in the process. Many of us can also (2) use green energy. This creates many more American jobs than using fossil fuels does. It cuts our dependence on foreign oil (and tames our foreign debt). Some of us can (3) remove carbon from the air with grazing and tilling practices on farms, ranches, and gardens.
What We Must Do
Stop putting carbon in the air.
Take our legacy emissions out of the air.
Screen out some sunlight, until we return CO2 and CH4 to safe levels.
Slide Shows, Videos & Library
Dr. Fry's scientific paper deriving the results shown above was published in Decmber 2020 in Environmental Sustainability and Climate Change. It is here:
Albedo Changes Drive 4.9 to 9.4°C Global Warming by 2400. PDF, 15 pages
The main presentation (slide show) summarizes global warming - the evidence, the impacts, and what to do. The paper above was developed out of research summarized in the slide shows. The slide show (updated most months) is available in PPTX and PDF formats.
Global Warming - So What? PPTX (Office 2011) Last updated September 2023.
This set of 100 slides takes 86 minutes to view. Many of the slides are dense with information, so information on slides generally rolls out a bit at a time. When a black dot appears in the upper right corner of a slide, click to continue.
(The slides works well with PowerPoint, but the many graphs, plus timing and direction of text roll-out, get mangled with Keynote. Open Office is also missing text color.) If you cannot use PowerPoint, try the PDF version, available here:
Global Warming - So What - PDF
Global Warming - So What? - PPT (older Office), with 82 slides is also available. It was last updated in April 2017. (When a recent version of Office (PPTX) opens an older version (PPT) file, it declares an “error.” Formatting changes it makes to “repair” the file create a few vertical mis-spacings in complex text.)
Much of the same information is here (Global Warming - So What - DOC), a 2016 scholarly paper with references, an abridged version of the slide show at that time, with less detail than the Lite version but more than the Mini one. It does not include much of my albedo research and projections of the future.
Additional slides, with further details, are available here: Additional Slides. These 34 slides take almost half an hour to view.
Shorter slide shows summarizing the situation are also available.
They have most, much, or some of the same information.
Global Warming - So What? - Lite (PPTX) has 81 slides. It takes 66 minutes to view. It omits many details. Last updated Janury 2023.
The PDF version is here: Global Warming - So What - Lite - PDF.
Global Warming - So What? - Mini (PPTX) has 69 slides and takes 52 minutes. It also omits explanations of warming. Updated Janury 2023.
A 15-minute presentation (28 slides) from August and October 2019, oriented toward the science, is available here:
Climate Feedbacks - Why We Must Go Far beyond Carbon Neutral
Another version (39 slides, 23 minutes), from an international conference in September 2022, is here:
Dominant Role of Albedo Feedbacks in Recent and Future Warming.
A quite similar version (43 slides, 23 minutes), from an international conference in August 2022, is here:
Key Role of Albedo Feedbacks in Recent and Future Warming.
The presentation is developed in more scientific depth below, but with less text and diagrams per slide.
Feedbacks - Why Carbon Neutral Is Way Not Good Enough
Its 92 slides take 38 minutes to view.
Below are 5 videos of the PPTX slide shows.
Immediately below is a video version with 67 slides that takes 42 minutes. It was developed for a class of high school science teachers and has more filler material, such as tables of contents, recaps, etc. Click on the diagram “Albedo Effect” below. To the right are fuller video versions of the same story. One is a 70-slide version video (48 minutes). Below that is a still fuller 58-minute version.
. A presentation with updated projection through 2400, developed for an audience at the Massachusetts Institute of Technology in October 2018 is available below as slides, not video.
Phasing Out Fossil Fuels by 2050 Can Hold Global Warming below 8°C.
Its 43 slides take 26 minutes to view. As shown above, slower phase-out (2035 peak) goes with an estimated 9°C warming by 2400, while phase-out by 2050 combined with massive CO2 removal, mostly later this century, can hold warming by 2400 to less than 3°C. 2°C warming is probably not possible without 4 things simultaneously: (1) rapid phase-out of fossil fuels, (2) massive CO2 removal - soon, (3) refreezing the Arctic Ocean, and (4) ongoing injections of aerosols or particulates into the stratosphere.
Albedo Changes Drive 4.9 to 9.4°C Global Warming by 2400 is a 35-slide video for an August 28-29, 2020 conference from London. 25 minutes. It adds my findings on climate sensitivity.
Albedo Changes Drive 4.9 to 9.4°C Global Warming by 2400 - is a 42-slide show for an October 24-27, 2020 conference from MIT and Beijing. It adds my research on warming trends since 1975 (see mid-page above) across the US.
… The older presentation below focuses on lessons that climate change several million years ago hold for Earth’s climate in the next few decades to centuries. Written for the educated public, including scientists, its 35 slides take 24 minutes to view. To model the future to 2400, it emphasizes changes in how much sunlight reflects from Earth’s surface and atmosphere (clouds, haze), plus natural emissions from permafrost, etc. Emissions since this show was developed (400 ppm CO2, vs 419 in 2021) commit Earth’s land surface to more warming than shown in the title.
4.5 to 7.8°C Global Surface Warming from Today’s CO2 and CH4 Levels - Note the earlier data, when CO2 and CH4 levels were lower and not quite as much future heating (4.5 or 7.8, versus 4.9 or 9.4) was baked in.
Climate Change in the Distant Past Shows Our Path to Our Hot Future - for audience in India. It was updated (50 slides takes 31 minutes) and adapted for an international audience at the Massachusetts Institute of Technology.
The presentation below focuses on lessons that climate change in earlier eras holds for Earth’s climate in the next few decades to centuries. Written for a broader audience, its 31 slides take 20 minutes to view.
The 26-slide presentation below gives more detail on temperature trends in the US. It takes 13 minutes, plus appendices (54 slides) with details for each region and state. US City Summers Are Warming 10.5°F / Century
A set of key slides is here: 14 Key Warming Slides.
Slides for Massachusetts Citizens Climate Lobby - 30 slides, 12 minutes, April 2021. It emphasizes the coming rapid decarbonization of electricity production and ground transportation, due to economic forces of innovation in batteries, solar and wind.
It draws on Tony Seba’s work" www.youtube.com/watch?v=duWFnukFJhQ&t=2097s . Tony’s Seba’s work is a must-watch for understanding our emission trajectory for the next 1-2 decades.
19 more web pages address aspects of climate change. They include excerpts from the slide shows, plus many other graphs, maps, tables, diagrams, and explanatory text. 5 pages show how climate is changing, starting with Overviews. 1 page is about the carbon cycle. 5 pages detail impacts, starting with Food. 3 pages cover energy, starting with "Coal, Oil, Gas, Nuke”. 2 pages cover emissions, pricing, CO2 levels, politics of warming, investment, and so forth. Two pages are data and one is about me.
These pages contain several books, plus some 16,000 articles, with 1,500-2,000 illustrations (some multiple panel, some duplicate), most drawn from the articles. Most pages have 6-18 sections, each with 7 to 250 articles (files) and usually some illustrations. (Over 1/3 of the ~15,000 articles appear in more than one place, as relevant.) Links to other sections on a page appear above and below the sections. Links to other pages appear atop each page.
Some 350 articles in PDF form are original scientific studies or summary studies (e.g., IPCC report pieces or US National Climate Assessment chapters). But about 16,000 are summaries of more studies (RTF format, plus a few in DOC), drawn from the scientific press. About 500 more files are small data bases in XLS (e.g., daily Arctic Ocean ice area; weekly US drought indices; monthly US electricity by source / fuel; annual CO2 emissions by nation; mean of daily summer highs, by US city by month and year; etc).
Dr. Gene R. H. Fry