This week an interesting paper was published in Geophysical Research Letters by climate modelers at the National Center for Atmospheric Research. They estimated future temperature changes if the atmospheric concentration of carbon dioxide (CO2) were held constant at current levels (well, actually 2000 levels). The results are shown in Figure 1.
Figure 1. Global average temperature change projected from 16 different climate models for the 21st century if atmospheric CO2 levels are held constant at the year 2000 levels.
The resulting temperature change by the year 2100 is around 0.5°C, and it basically represents the thermal inertia of the coupled land/ocean/atmosphere system. It works out to about 35 years.
(Readers of these pages will recall that we achieved the same number without a climate model way back in 2001. The related scientific reference is at the end of this article.)
So all we have to do is cut emissions back to 2000 levels, right?
Wrongo. This result holds only if atmospheric concentrations of greenhouse gases remain at 2000 levels, which is impossible.
Figure 2 shows why. Figure 2a gives the annual global emissions of CO2, with year-over-year increases for the past 50 years (and for many years prior to that—for the complete record see here . Figure 2b is the annual change in the atmospheric concentration of CO2 (as measured atop Hawaii’s Mauna Loa volcano). There is a slow rise in the rate of atmospheric CO2 concentration increase (although it is barely significant over the past 25 years). Cutting back on emissions doesn’t result in much of a slowdown in the rate of atmospheric CO2 build-up.
Figure 2 (a). Annual global emissions of carbon dioxide; (b) Annual increase in atmospheric carbon dioxide concentration; (c) Annual global emissions of carbon dioxide expressed as a percentage reduction from the 2002 emissions level.
Figure 2c shows the global CO2 emissions as a percentage of the 2002 value. This chart allows each of us to dial back the global CO2 emissions to any point, and then to see what the associated rate of atmospheric CO2 accumulation would be. From there, we can calculate the future temperature. To make this easier for everyone, we created Table 1.
(note: A full adherence to by all the nation’s of the world to the terms of the Kyoto Protocol would, by the year 2010, reduce emissions to about a 2005 level and then rise from there. Kyoto doesn’t even make it into this chart, because it does nothing.
Table 1. Dial in your own climate change. Pick an emissions reduction (from 2002) levels (column 2) and see what year in the past that it equates with (column 1) and then see the temperature change by 2100.
For example, let’s cut the world’s CO2 emissions by 10 percent in the next couple of years (actually this really isn’t reasonable at all—as Figure 2a shows, the annual rate of CO2 emissions continues to grow—so unless a cutback is achieved immediately, a 10 percent reduction quickly becomes 12 percent, which quickly becomes 15 percent in just a few years). From Figure 2c we see that a 10% reduction gets us somewhere to around the emissions levels of 1991. And from Figure 2b, we see that 1991 levels of CO2 emissions resulted in an annual atmospheric CO2 concentration increase of about 1.6 ppm/yr. From Table 1, we see that by the year 2100 that this increase will result in CO2 levels of about 530ppm and an approximate temperature rise of about 1.47ºC by 2100. (for reference, a projection based upon the current rate of CO2 build-up is about 563 ppm by 2100 and a temperature rise of about 1.73ºC).
What about a 50% reduction in global CO2 emissions? That takes us back to the levels of the late-1960s, which were associated with about a 1ppm/yr CO2 increase. Starting from today’s level of about 380ppm, in 94 years at 1ppm/yr we get about 474 ppm and an associated temperature rise of about 1.01ºC. In other words, by reducing global emissions of CO2 by about 50% from 2002 levels you prevent about 0.7ºC of warming. That is not a lot of bang for quite a lot of bucks!
Even a 10% reduction is not easy. A full adherence to the Kyoto Protocol will lower global CO2 emissions by about 8% from the expected levels in 2010. 8% below 2010 levels is still several percentage points above the current 2002 levels. And, it is looking more and more like no major emitting nation is going to meet their individual emissions targets, let alone the world.
The nickel-and-dime emissions reductions that will be vehemently fought over in Congress next January will have no effect on future climate. Only a major break-through in energy production technology (or a massive turn-over to nukes) will result in a lowering of global CO2 emissions to the degree necessary to impact our future climate pathway in any meaningful way. That time will come, but it isn’t yet upon us. And the more money we waste on futile attempts to stop emissions, the longer away that day is.
Michaels, P.J., 2001. Integrated Projections of Future Warming based upon Observed Climate During the Greenhouse Enhancement. 1st Intl. Conf on Global Warming and The Next Ice Age, American Meteorological Society, Halifax NS, 162-167.
Teng, H., et al., 2006. Twentiy-first-century climate change commitment from a multi-model ensemble. Geophysical Research Letters, 33, L07706, doi:10.1029/2005GL024766.