One of the biggest challenges about reporting on climate change is scale. An example: the world has come to agree that we should work to keep global temperatures from rising more than 3.6º F (2º C) above levels that were normal before we began pumping carbon dioxide into the atmosphere in the 19th century. (Temperatures have already risen by about 1º F.) But what does a 3.6º F change really mean to the average person? On Wednesday in New York City, the temperature ranged between 55º and 66º F, and it felt like a completely normal day. (In fact, it was almost exactly average—historically temperatures in New York City on Oct. 9 have ranged between 53º and 63v F.) If we’re accustomed to handling much bigger temperature variations on an average autumn day—let alone the weather changes from summer to winter—it shouldn’t be hard to adapt to a climate in which the average temperature is just 3º F or so warmer than average, right?
Wrong. As an important new study published in Nature shows, even what seems like a small increase in the average air temperature could have a major effect on the planet—and on our well being. After crunching data from existing climate models, researchers from the University of Hawaii at Manoa, led by the geographer Camilo Mora, calculated 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 at those places in any year between 1860 and 2005, the span of years for which scientists have the most reliable records. By about mid-century, the coldest year will be warmer than the hottest year in the past.
For some regions, especially in the tropics, that tipping point will come much earlier, with Mexico City experiencing the transition as early as 2031, Lagos by 2029 and Bogota by 2033. That’s because the natural variation in air and ocean temperatures in the tropics tends to be very small, so even a seemingly inconsequential change is enough to push the climate well out of the historical norm, leading to average temperatures much hotter than even these sizzling regions are accustomed to. Given that more than 3 billion disproportionately poor people live in the tropics and subtropics, we should be very worried about how they’ll manage to adapt to a climate hotter than they’ve ever experienced. “Our results suggest that the countries first impacted by unprecedented climates are the ones with the least capacity to respond,” said Ryan Longman, a study co-author and graduate student at Hawaii, in a statement. “Ironically, these are the countries that are least responsible for climate change in the first place.”
But if human beings in the tropics will struggle to adjust to the new normal, plants and wildlife will face an even tougher challenge. This is because of what’s known as Rapoport’s rule: the closer they live to the equator, the smaller the environmental range most species can tolerate. Tropical wildlife are adapted to very specific, very narrow tropical climates, which is also why the tropics support such an array of biodiversity. But if that climate should change, tropical species will have a very difficult time adjusting. So difficult, in fact, that many will likely go extinct. The Nature study projects that the climate tipping point will occur in marine and terrestrial biodiversity hotspots—the top 10% of the planet in terms of richness of species—at least a decade earlier than the global average. “Conservation practitioners take heed—the climate-change race is not only on, it is fixed, with the extinction finish line looming closest for the tropics,” wrote Eric Post a biologist at Pennsylvania State University, in an accompanying article in Nature.
The good news is that these changes aren’t inevitable. The Nature study notes that if global greenhouse gas emissions can be stabilized—which is still a long way from happening—we can delay the point of “climate departure,” when the coldest years are hotter than our hottest years, by more than two decades. There’s also the caveat that the Nature projections are based on climate models, complex programs that try to forecast how the planet will respond to changing levels of greenhouse gases. The climate system is incredibly complicated, and the further out scientists try to project, the less certain they can be about the outcome, which is why the Intergovernmental Panel on Climate Change (IPCC) gives such wide temperature ranges for future climate change. (You can see a full list of projected years for climate departure in different cities here.)
Of course, the flip side of the possibility that climate models are overestimating global warming is that they could well be underestimating it instead. If the projections made in the Nature paper hold true, many of those of us alive today will experience the climate change tremendously. The hottest year in U.S. history was 2012, featuring brutal droughts and heat waves. By mid-century, we could look back upon that year as pleasant and cool—at least comparatively. A few degrees change can make a tremendous difference.