We Still Have Time to Avoid Dangerous Climate Change

This post is by James Wang, Ph.D., a climate scientist at Environmental Defense.

A study by Weaver et al., published this month in the journal Geophysical Research Letters, reports that "All emission targets considered with less than 60% global reduction by 2050 break the 2.0°C threshold warming this century…." (They mean 2.0°C above the pre-industrial temperature, equivalent to 2.3°F above today’s temperature. For more on threshold temperatures, see “How Warm is Too Warm?“) Even more disturbing, they say, "Even when emissions are stabilized at 90% below present levels at 2050, this threshold is eventually broken."

That makes all our efforts seem hopeless. But are they right? In a word, no. Here’s why.

Weaver’s pessimistic results arise from two assumptions, with the first being especially crucial:

1. CO₂ emissions are held steady after 2050, and
2. Atmospheric levels of all non-CO₂ greenhouse gases and aerosol particles are held steady at current levels.

These assumptions are downright peculiar. There is no reason to expect that CO₂ emissions reductions would not continue after 2050, and all of the major policy options on the table would require reductions of other greenhouse gases, as well.

More realistic assumptions show that warming can stay below the 2.3°F threshold. In Avoiding Dangerous Climate Change (Schellnhuber et al, eds.), studies by Meinshausen and others calculate a roughly 50 percent likelihood that warming can be kept below 2.3°F if the atmospheric concentration of greenhouse gases is stabilized at 450-500 parts per million (ppm) equivalent CO₂. To achieve this result requires that:

• By 2050, global greenhouse gas emissions, including that from deforestation, are 25 to 60 percent below the 2005 level. (Wealthy, high-emitting countries like the U.S. can and should cut emissions by 80 percent. See Bill’s previous post, U.S. Emissions Target, for why.)
• Emissions continue to decline after 2050.

Their assumptions are much more reasonable than those by Weaver et al., and their conclusion is much less gloomy. The Weaver article suggests using technology to capture CO₂ from the atmosphere. That would be useful if there were a cost-effective way to do it, but it’s not our only option. A cap on greenhouse gas emissions would be very effective.