By the end of the next decade, greenhouse emissions must be a quarter lower than they were last year to enable humanity to keep the world below 2 degrees Celsius of warming by 2100. Otherwise, the only option to avoid breaching that threshold would be an extremely expensive crash decarbonization program. To keep the world below 1.5 degrees would require cutting emissions by more than half by 2030.
“Had serious climate action begun in 2010, the cuts required per year to meet the projected emissions levels for 2°C and 1.5°C would only have been 0.7 per cent and 3.3 per cent per year on average,” the authors found. “However, since this did not happen, the required cuts in emissions are now 2.7 per cent per year from 2020 for the 2°C goal and 7.6 per cent per year on average for the 1.5°C goal.”
Meeting either target would require much more effort than countries pledged at the 2015 Paris climate conference. If nations met their Paris commitments — hardly a given as major emitters such as Australia, Brazil, Japan and the United States fall behind — that would give the world a 66 percent chance of limiting warming to 3 degrees Celsius by 2100.
Such an outcome would be better than uncontrolled warming. But it would not be pleasant. Even at 2 degrees, coral reefs would disappear, rising seas would threaten millions, mosquito-borne diseases would proliferate, droughts and deluges would stress people and landscapes, and tropical fisheries would empty. The risk of triggering feedback loops — for example, thawing permafrost resulting in the release of massive amounts of currently trapped greenhouse gases — would escalate. No one should want their children or grandchildren to live in that world.
To the extent there is a bright side, it is that the cost of renewables has plummeted in recent years, to the point that new solar and wind power is cost-competitive with fossil fuel sources of electricity. But replacing the old fossil fuel plants that power the world is still a massive task. Moreover, greenhouse gases released in the agricultural, industrial and, especially, the transportation sectors require far more effort than countries are applying.
Experts have known for years what the United States must do: place a strong and steadily rising price on carbon dioxide emissions, invest heavily in clean-energy research and development, and make climate a top priority in international diplomacy. President Trump is instead denying the problem.