And it's a good thing, because I haven't gotten past Exhibit A of this paper and my heart is already thumping with fear.
Exhibit A concerns the atmospheric level of (GHGs) over the last 800,000 years. Ice core drilling in Antarctica began in the late 1970s and is still ongoing. The record of carbon dioxide (CO2) and methane (CH4) trapped in tiny ice-core bubbles currently spans 800,000 years.2 It is important to recognize that the numbers in this unparalleled 800,000-year record of GHG levels are among the very best data that exist in the science of paleoclimate. Almost all other data (including past temperatures) are inferred indirectly from proxy variables, whereas these ice-core GHG data are directly observed.
The pre-industrial-revolution level of atmospheric CO2 (about two centuries ago) was 280 parts per million (ppm). The ice-core data show that CO2 varied gradually during the last 800,000 years within a relatively narrow range roughly between 180 and 280 ppm. Currently, CO2 is at 385 ppm, and climbing steeply. Methane was never higher than 750 parts per billion (ppb) in 800,000 years, but now this extremely potent GHG, which is 22 times more powerful than CO2 (per century), is at 1,780 ppb. The sum total of all carbon- dioxide-equivalent (CO2e) GHGs is currently at 438 ppm. Even more alarming is the rate of change of GHGs, with increases in carbon dioxide hardly ever exceeding 30 ppm over any past thousand-year period, while now CO2 has risen by 30 ppm in just the last 17 years. Thus, anthropogenic activity has elevated atmospheric CO2 and CH4 to levels far outside their natural range at a very rapid rate. The unprecedented scale and speed of GHG increases brings us into uncharted territory and makes predictions of future climate change very uncertain. Looking ahead a century or two, the levels of atmospheric GHGs that may ultimately be attained (unless decisive measures are undertaken) have likely not existed for tens of millions of years and the speed of this change may be unique even on a time scale of hundreds of millions of years. We seem headed for a unique planetary experiment of subjecting the Earth's system to an unprecedented shock by geologically-instantaneously jolting atmospheric stocks of GHGs far above their highest level over the last several million years. We simply do not know what will happen under such extreme circumstances.