It’s the debate du jour in the climate blogosphere: How much hotter will the planet get as carbon dioxide levels in the atmosphere rise? The contention began July 20, when the Economist reported that a yet-to-be-released table of data from the Intergovernmental Panel on Climate Change (IPCC), showed that global temperatures would not rise as high this century as scientists had previously predicted.
For years scientists have said that if atmospheric levels of carbon dioxide reach 450 parts per million (ppm) the planet would heat up by an average of 2 degrees Celsius above pre-industrial levels. They have also said that if the world crosses that threshold, ecosystems worldwide would suffer serious damage. The IPCC data, however, indicated that levels of 425 to 485 ppm would correspond only to a temperature increase of 1.3 to 1.7 degrees C by 2100.
That’s a big difference. The Earth has warmed by about 0.8 degree C since the start of the Industrial Revolution, when the CO2 count was 280 ppm. This past May the Earth hit the 400-ppm mark for the first time in 800,000 years, and the average level is now climbing at about 2 ppm annually. Falling short of 2 degrees C by 0.3 degree or more could delay damaging effects. However, the notion of diminished warming is dubious at best. If anything, the world could hit the 2-degree threshold sooner than it thinks.
After the Economist published the table, which it had somehow obtained, IPCC spokespeople and independent experts quickly trashed the data—and the magazine itself. They said the table was a preliminary draft, was not reviewed, was not even being used by the relevant IPCC experts, and should never have been quoted.
The IPCC stands by its equation of 450 ppm = 2 degrees C, based on “climate sensitivity” data—how much temperatures will rise given an ongoing increase in atmospheric carbon dioxide. And indeed, prior IPCC temperature projections have held up. Those projections matter because the desire to avoid warming of more than 2 degrees C has driven policy decisions in many countries and underlies international climate agreements.
Those agreements notwithstanding, global greenhouse gas levels continue to rise. Industrialized countries actually cut their CO2 emissions by 7 percent in the past five years, according to the Worldwatch Institute, but emissions from developing countries have soared. The total global output keeps increasing, hitting a new record high of 31.6 million metric tons in 2012.
Furthermore, recent science shows that two major mechanisms that tend to slow the rise of atmospheric CO2 are diminishing. First, over the past decade the oceans have absorbed about 26 percent of all CO2 emissions, according to the Scripps Institution of Oceanography. About 28 percent is taken up by plants, leaving 46 percent wafting into the atmosphere. But scientists from various institutions have noted that the rate of ocean absorption is slowing. There are several reasons why, but part of the effect is simply that the more the water absorbs, the harder it becomes to store even more. The atmosphere will likely take up a larger share of emissions—unless we plant lots of trees.
Second, as the world shifts away from burning coal and reduces tailpipe emissions, those steps lessen the amount of aerosols—tiny particles—released into the atmosphere. We benefit with less pollution and fewer health problems, but aerosols also block incoming sunlight; fewer particles will allow more solar heating.
Some experts point out that we’ve already hit the 450-ppm mark. Technically speaking, the level is called “450-ppm equivalent”—the warming from all greenhouse gases, such as methane, expressed as if all the heating were due to CO2. Estimates indicate that when CO2 levels alone reach about 400 ppm, the total effect of all gases would be about 450-ppm CO2 equivalent. Having hit 400 ppm of CO2 for a brief time in May (the high point each year), we’re essentially at that 450-ppm equivalent now.
It is still difficult to say how much temperatures will rise by 2050 or 2100 due to the carbon dioxide that is already in the atmosphere—known as “the warming in the pipeline.” There is a lag between any rise in CO2 levels and the heating that results, so the planet is “locked in” to further warming—and to the chief repercussions such as further sea level rise. But the IPCC has released good estimates of the pipeline: the best case is that the average global temperature at the Earth’s surface will rise 1.5 degrees C by 2100, compared with 1990 levels. The worst case is 4.5 degrees C, and the most likely case is 3 degrees C.
In his own assessment of the numbers, Dana Nuccitelli, a physicist who writes at the Skeptical Science blog—known for deep analysis of these matters—notes that the 1.5 degrees C case would only be possible if the world stopped increasing emissions by 2020 and then began reducing them by 3.5 percent a year. As he notes, that scenario “involves extremely aggressive greenhouse gas emissions reductions.”
Aggressive is not impossible, although it may be unlikely. Still, anything that the world does to reduce emissions slows the eventual temperature rise. Scientific American has written about many possible steps, including a succinct short-list by my colleague David Biello.
We’ll have an updated—and fully peer-reviewed—analysis of warming soon; the IPCC will release its next big assessment of CO2 data and all things climate on September 23.