A Glimpse at Our Possible Future Climate, Best to Worst Case Scenarios
Posted on 13 February 2013 by dana1981
Recently there has been widespread discussion that perhaps the Earth's climate is not quite as sensitive to increasing atmospheric CO2 as climate scientists previously believed, which would be good news, because it would give us more time to reduce human greenhouse gas emissions before the worst climate change impacts are triggered. The case for a most likely equilibrium climate sensitivity of around 2.5 °C average surface warming in response to a doubling of CO2, as opposed to 3 °C, is not yet very compelling, but it is certainly a possibility. In fact, the value could very plausibly be anywhere between 2 °C and 4.5 °C.
This begs the question, what might the future climate look like in best case, most likely case, and worst case scenarios? To answer this question, we will examine how much warming we can expect under various human greenhouse gas emissions scenarios if the real-world equilibrium climate sensitivity turns out to be 2 °C (best case), 3 °C (most likely case), or 4.5 °C (worst case). There is a relatively small chance that the sensitivity could be lower than 2 °C or higher than 4.5 °C, especially if we consider very long timescales in which slow feedbacks can kick in, and the so-called "Earth System Sensitivity" may be in the range of 6 °C surface warming in response to doubled CO2.
Nevertheless, for our purposes here we will limit ourselves to the 2–4.5 °C likely equilibrium sensitivity range. But first we have to investigate at what temperatures we expect various climate consequences to be triggered.