| 英文摘要 | In his news item “Global warming forecasts sharpen” in the Breakthrough of the Year news package (18 December, ), P. Voosen wrote that new research on climate sensitivity “rule[s] out” the most dire climate scenarios. Although this work is a major advance, its scope is insufficient to eliminate the worst possible outcomes.
The research to which Voosen refers constrains “climate sensitivity”—the warming response to a given increase in the atmospheric CO2 concentration ([ 1 ][1]). However, climate sensitivity does not account for the complex relationship between human CO2 emissions and changes in atmospheric CO2. That relationship depends on carbon cycle feedbacks, which currently act to stabilize climate but could weaken or reverse as warming progresses ([ 2 ][2], [ 3 ][3]). Even uncontrolled warming is potentially possible ([ 4 ][4]); paleoclimate evidence suggests that carbon cycle feedbacks may have amplified past natural warming trends ([ 5 ][5]–[ 7 ][6]). Of particular concern are greenhouse gas emissions from thawing permafrost, which are substantial now and will only increase as warming progresses ([ 8 ][7]).
Because of limited scientific understanding, carbon cycle feedbacks are incompletely represented in many climate models, including those used in the recent Intergovernmental Panel on Climate Change special report, “Global warming of 1.5°C” ([ 9 ][8]). Every ton of carbon emitted by permafrost is one less ton that humans can emit while limiting warming to 1.5° or 2°C. Thus, unaccounted-for carbon cycle feedbacks increase the challenge of meeting any top-line climate goal.
The new constraints on climate sensitivity that Voosen highlights are welcome, but they must be carefully interpreted to avoid creating a false sense of security about the possible severity of the warming response to human emissions of greenhouse gases. Even with perfect knowledge of climate sensitivity, we would still need better understanding of carbon cycle feedbacks to determine how much humans can emit and still meet top-line climate policy goals of limiting global warming to 1.5° or 2°C—a question of fundamental importance to our collective future.
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