New research shows that if the level of greenhouse gas emissions is not curbed, temperatures across the Earth will rise to levels that have no recorded precedent by the middle of this century. The predictions come from calculations by scientists from the University of Hawaii at Manoa, who found that by 2047 (plus or minus five years), the average temperatures in each year will be hotter across most of the planet than they had been in those locations in any year between 1860 and 2005. This means that for a given geographical area, the coldest year in the future will be warmer than the hottest year in the past. These unprecedented temperatures will arrive in the tropics before other areas, putting increased stress on the rainforests and coral reefs found there.
The research is based on climate models, which are huge computer programs that reproduce the physics of the climate system and then predict its response to greenhouse gases. Climate models are the best tools available for predicting climate change, though there are acknowledged problems. No one can be 100 percent certain at their accuracy at predicting many decades ahead.
The models did show that these unprecedented temperature changes could be delayed by 20 to 25 years if there is a concerted global effort to bring greenhouse gas emissions under control. This delay would give nature and human society more time to adapt and would allow humans to develop technologies that could further reduce emissions. The alternate scenario with greenhouse gas emissions stabilized gave the global mean climate departure date as 2069.
Dr. Mora, the chief author of the paper, assigned his class of graduate students to analyze forecasts produced by 39 of the world’s foremost climate models. The models’ results are publicly available and are financed largely by governments. Instead of looking at average temperature changes across the planet, Dr. Mora and his students divided the Earth into a grid with each cell representing 621 square kilometres. The results from the 39 climate models were averaged and from this result they calculated a date they called “climate departure” for each location, which is the date after which all future years are predicted to be warmer than any other year in the historical record for that particular spot on the globe.
If greenhouse gas emissions remain high, more than half the Earth’s surface after 2047 will experience annual climates hotter than any other year between 1860 and 2005, which were the years historical temperature data and reconstructions are available. If the current rate of carbon dioxide emissions continues, then most of the world’s ecosystems will be pushed into climatic conditions they have not experienced for many millions of years. This date could be pushed back to 2069 if concentrated efforts are made to bring the rate of emissions down.
The technique used in the analysis meant that the team were able to specify climate departure dates for individual cities. Using high emission rates, climate departure will start in 2020 with Manokwa, Indonesia. Then Kingston, Jamaica. Within the next two decades, 59 cities will be living in what is essentially a new climate, including Singapore, Havana, Kuala Lumpur and Mexico City. New York City will experience the change in 2047, plus or minus the five-year margin of error. Lower emissions would push the climate departure to 2072. For Beijing, climate departure would come in 2046 under high emissions, and 2078 under lower emissions. The dates for Moscow are 2063 and 2092; for Washington, 2047 and 2071. Australia has dates ranging from 2038 in Sydney to 2049 in Adelaide. The last of the 265 cities to move into their new climate will be Anchorage, Alaska, in 2071.
Climate variability in the tropics is smaller than in high latitudes so the extra heat trapped there by greenhouse gases will push the temperature beyond the historical bounds sooner than anywhere else. Under high emissions, the climate departure date for Mexico City is 2031; 2029 for Jakarta and for Lagos, Nigeria, and 2033 for Bogotá, Colombia. Previous studies have already shown that corals and other tropical species that currently live in tropical areas are near their physiological limits.
Earth has already crossed the threshold into an entirely new regime in terms of ocean acidity. That happened in about 2008, with every year since then more acidic than the old record.
Though climate change appears to most people to have the most serious effects at the poles, the Mora paper shows the biggest risk to nature and to our society in the near term may be in the tropics. The tropics will experience an unabated heat wave that will threaten biodiversity and heavily populated countries that have the fewest resources to adapt.