I am reprinting and updating one of my first posts (from August 2006) because I realize the subject remains confusing to people. Indeed, I myself recently wrote a post on projected warming by 2100 that failed to explain whether I was talking about warming from recent levels or from preindustrial levels. And, as I’ll blog on shortly, a recent statement I made on projected U.S. warming by 2100 was erroneously attacked by people unaware of basic climate science — the vast majority of this country is projected to warm up much faster than the planet as a whole, as the figure depicts for a scenario of 2.8°C warming from 1990 to 2095.]
Many of the articles that discuss the projected temperature change from global warming do not explain crucial points in a clear fashion. A clarifying note is in order since Climate Progress discusses temperature a great deal — how much we are likely to warm on our current emissions path, why the planet must stay as close to 2°C (3.6°F) warming, and how we can do so.
Americans use the Fahrenheit scale of temperature and have the most intuitive familiarity with it. CP will also use the Celsius scale because most scientific research uses it. Anyone who wants to become knowledgeable about global warming research needs to become familiar with thinking in Celsius terms. Since our focus is on temperature change, here is the key conversion:
A 1°C change equals a 1.8°F change.
Thus a 5°C change equals a 9°F change
Different parts of the globe are expected to warm up at faster or slower rates than the global average. The land typically warms up faster than the oceans, and higher latitudes warm up faster than the tropics. Most of the inland continental United States is expected to warm up roughly 40 percent faster than the global average — see, for instance, the IPCC’s Fourth Assessment Summary for Policymakers figure SPM.6 (reprinted above), which shows the geographic pattern of surface warming 2090-2099 in the (increasingly) unlikely event we are able to keep total planetary warming to 2.8°C (from 1980-1999 average).
The Arctic, of course, is projected to warm up the most (see “What exactly is polar amplification and why does it matter?“) — some 2.5 times the average global warming. Already, Greenland has been warming up more than 2 times faster than the planet.
So if the median warming projection is another 4°C to 5.5°C (about 7°F to 10°F) from 1990 levels as the latest science projects (see “An introduction to global warming impacts: Hell and High Water“), that would mean an additional average warming of some 5.6°C to 7.7°C (10°F to 14°F) over most of the continental United States. Alaska and Greenland would probably warm closer to than 8°C to 11°C (about 15°F to 20°F).
It’s going to get very, very hot on business as usual path. That’s why I prefer “Hell and High Water” to global warming.
A third confusion can arise when scientists report how much warming will result from a rise in greenhouse gas concentrations. Some report how much the temperature will rise from pre-industrial temperature levels (which is the most important from a perspective of total impact) while many studies talk about how much additional or further warming will occur by 2090-2099 or from the 1980-1999 average, while still others talking about warming from present-day levels. The Earth has already warmed 0.8°C through 2005, so the difference is significant. CP will try to state which it is using each time [which I failed to do in my original version of "An introduction to global warming impacts: Hell and High Water" and the post on MIT projections -- but now I have fixed that].
The UK’s Hadley Center now predicts median warming from pre-industrial temperature levels of 5.5°C by 2100 (see here). MIT’s Joint Program on the Science and Policy of Climate Change now predicts median warming of 5.1°C by 2100 (see here) from the 1980-1999 average. So you can add some 0.5°C and 1.0°F to the MIT numbers for comparison with pre-industrial temperatures.
That is, both Hadley and MIT’s median projection is some 5.5°C from preindustrial levels. For most of the inland States, that is about 14°F warming.
Since it is total warming from preindustrial levels that determine the impact — since we are trying to stabilize as close as possible to 2°C total warming from preindustrial levels — CP’s default will be warming from preindustrial levels.
A final, related confusion: Some scientists report how much the temperature will rise ultimately (due to a given rise in greenhouse gas concentrations) while others report only how much the temperature will rise by 2100. Because of the lags in the climate system, those figures can be quite different.
The IPCC’s Fourth Assessment Summary for Policymakers notes that for the scenarios that stabilize late in the century at high levels late in the century, “assessed models project that about 65 to 70% of the estimated global equilibrium temperature increase, assuming a climate sensitivity of 3°C, would be realised at the time of stabilisation. For the much lower stabilisation scenarios, the equilibrium temperature may be reached earlier.”
CP will usually describe how much temperature is projected to rise by 2100.
I hope that clears things up!