What's in a half degree? 2 very different future climates

A half-degree barely could make the difference between ice-free Arctic once a decade and once a century; between coral reefs being almost completely annihilated and up to 30% suspended; and between one-third of the world's population exposed to extreme heat waves and one-tenth.

These new scenarios were defined last week in a new report by the Intergovernmental Panel on Climate Change (IPCC) that explores the possibility of limiting the rise in global temperature to 1.5 ° C above the pre-industrial era by 2100, instead of the upper limit of 2 degrees C agreed in the historic Paris Agreement three years ago. The report exposes the closing of the window to which humanity must choose the future it wants.

Preventing a temperature rise of 2 degrees Celsius will be a major challenge that current commitments from various countries are likely to be unable to meet. And that is before Pres. Donald Trump withdraws the United States from the deal. However, the report states that a limit of 1.5 degrees C is not impossible, even if it will require immediate and drastic action, because the current rate of emissions would exceed this level between 2030 and 2052. The The most likely scenario to achieve this goal could be to exceed and then draw carbon dioxide from the atmosphere to bring the temperatures back into alignment.

While the perturbations of the climate system are already being felt with a warming of just under 1 degree Celsius, even the 1.5 degree C goal seems to have major negative effects. However, preventing another half-degree of warming would avoid entire ecosystems, cities and populations that are vulnerable to even greater damage. "We still have choices to make; we've already seen some of the peak damage, "says Kim Cobb, a climatologist at the Georgia Institute of Technology, author of the next major IPCC report.

Here are some climatic consequences that can be avoided if the heating is limited to 1,5 ° C:

Heat: The rise in average temperatures is clearly related to the frequency of heat waves and their severity. Studies have already shown the fingerprints of global warming on the major heat waves in today's climate, and the situation will only worsen as temperatures on warmer days rise faster than the world average. A study cited in the new report used climate models to see how the share of the world's population exposed to a heat wave (one with a 5% chance of occurring in a given year) would change. This number has increased from less than 10% of the current population to 50% with a warming of 1.5 ° C and more than 70% at 2 ° C.

ecosystems: Coral reefs have already been hard hit by warming and ocean acidification, with widespread bleaching in recent years. The reefs have one of the darkest results: a temperature rise of 2 ° C would eliminate 99% of today's reefs, while 1.5 ° C would save some of them, with losses included between 70 and 90%. Other animals face significant losses in places of life. The amount of climatically acceptable habitat lost by vertebrates and plants would double from a diet of 1.5 to 2 degrees C and triple for invertebrates.

Arctic: The Arctic has already warmed about twice as fast as the planet as a whole, causing thawing permafrost and melting sea ice. The 1.5 to 2 degree Celsius shift could mean an additional 1.5 to 2.5 million square kilometers of permafrost would be lost, while the Arctic Ocean would transition from ice-free conditions in the summer once every 100 years to once every 10 years.

Food and water: Warming temperatures also threaten the water and food sources on which humans depend. Allowing global temperature to increase by 2 ° C could double annual sea fish catch losses, increase the number of people exposed to water stress by 50%, and increase the decline in yields of major staple crops such as corn, rice and wheat.