The world is now around 1.4°C hotter than before the industrial era, and many climate impacts are already showing. Against that backdrop, scientists who set out the possible climate futures have dropped their most extreme one.
The seven new scenarios announced last week left out RCP8.5 and its successor SSP5-8.5, according to a piece published by The Conversation. In those worst-case storylines, nations make no effort to cut emissions and expand their use of fossil fuels. By 2100, carbon dioxide would almost triple to 1,135 parts per million, and the planet would run about 4.5°C hotter than in pre-industrial times.
The decision caught the attention of U.S. President Donald Trump. The article’s authors say he and other climate sceptics have read the removal as proof of failed modelling, or that climate change was a hoax. They reject that reading. Their view is that the scenario came out because climate efforts, slow and incomplete as they have been, have made a tangible difference. Solar, wind, electric vehicles and batteries have slowed the growth of emissions.
The authors are blunt that the job is far from done. Emissions are at record highs, and global warming is speeding up.
How the scenarios work
Scientists build a range of pathways for future greenhouse gas emissions from what has happened so far and what politics and technology might do over coming decades. They pick the pathways judged most plausible, then sample futures that are more or less optimistic about fossil fuel use. Research groups around the world run each scenario through different climate models, which produces data at global, regional and local levels.
The scenarios are not ranked by likelihood. All count as plausible. The spread in temperature outcomes by 2100, approaching 2°C between the most and least optimistic, shows how much depends on human choices.
The “8.5” in the old names refers to radiative forcing, the extra heat in watts trapped per square metre by 2100. Scientists had long argued over whether such a path was plausible at all.
What is left
None of the new scenarios is as pessimistic. The worst now envisions high emissions leading to warming of around 3.5°C by 2100, which the authors call still “very, very bad.”
The best case has also shrunk. The lowest scenario from the last round, SSP1-1.9, assumed strong action and rapid emission cuts, with warming peaking at around 1.5°C. Because global emissions have not yet begun to fall, the most optimistic new pathway peaks at about 1.9°C. The world will pass 1.5°C, they write, and the hope is only a temporary overshoot, followed by pulling carbon dioxide back out of the atmosphere.
Current policies and national actions point to around 2.6°C of warming by 2100, somewhere in the middle of the range. The authors give two reasons for revising scenarios regularly. Facts on the ground change: solar is rolling out far faster than expected, while fracking has opened large new fossil fuel deposits. And climate models keep improving, which sharpens projections of sea level rise and other impacts.
