Even though the Earth is warming, not every year is warmer than the year before. That's because natural climate cycles of warmth and cold are layered atop that human-caused warming trend.
Now, a new study lays out a date for when our warmed Earth will surpass the climate's natural variability and temperatures will consistently sit higher than they ever have, if humans keep emitting greenhouse gases at current rates.
That date is 34 years from now: 2047.
"We are now providing a new metric of when ongoing climate change will lead to environments like we have never seen before -- when the coldest year in the future will be warmer than the hottest year of the past," said Camilo Mora, a professor at the University of Hawaii, Manoa, and lead author of the study, released yesterday in the journal Nature.
But putting that date, which has an error margin of five years, on when the average spot on Earth will enter a new climate was not the only important finding of the research.
Mora and his colleagues also analyzed when many different parts of the world will see their temperatures go outside the historic range of temperatures recorded from 1860 to 2005.
Their analysis shows, somewhat surprisingly, that tropical climates will be the earliest to reach outside historical norms. This has big implications for the billions of people living in the tropics, as well as the huge set of tropical species and some of the most biodiverse places on Earth.
The reason for an earlier tropical impact is that the tropics do not experience as much natural variability in temperatures, explained Mora.
Why tropics will feel more heat first
If you think of natural temperature variability as an envelope of possible temperatures, in a place like Washington, D.C., that envelope stretches all the way down from a bitter February chill up to a sweltering July. The Arctic, which many think of as heavily affected by climate change, also has a wide range of temperatures. In the tropics, though, temperatures tend to sit inside a smaller envelope.
Because of that, climate change will push them outside of their range of variability earlier, according to Mora's analysis, which used data from 29 climate models.
From a biodiversity perspective, this is a concern, said Ryan Longman, a study co-author and a doctoral student at UH Manoa. "We know species have several ways of responding to rapid climate change," said Longman. They move, adapt or die.
Scott Doney, a senior scientist at Woods Hole Oceanographic Institution familiar with the paper, said this tie to biodiversity is a key aspect of the study.
"I think the important perspective from Mora and his colleagues is that of plants and animals and microbes. Where is the climate trend big relative to that natural envelope that organisms are used to?" said Doney.
Many of the tropical countries that will see early climate change are less developed, added Longman.
"Countries first impacted by unprecedented climates are those with the least economic capacity to respond. Ironically, these are countries that are least responsible for climate change in the first place," he said.
Alaska keeps its cool, for a while
This research is unique because, unlike most climate model research, which projects the dates at which average annual temperatures will rise by a certain amount, it outlines when climates of places around the world outstrip their historical variability.
"We noted that an element that had been less studied was the timing of climate change, which led us to develop our global index," said Abby Frazier, a student at the university and a co-author of the study.
The research team, which looked at the equivalent of a million maps and more than 330,000 species, included many students from UH Manoa. They also catalogued the dates at which other indicators like precipitation, ocean temperatures and ocean acidity will move outside of historical norms.
As Frazier noted, the acidity of the oceans has already surpassed natural variability. That occurred in 2008.
Those curious about when the climate in their part of the world will depart from the historical record can go to a basic interactive map created by the researchers. There, viewers can click on grid points on Warth and see when the climate at that locale will go outside those bounds.
The researchers also put together a listing of when the climates of different world cities will move outside historical variability. If humans do not curb emissions of greenhouse gases, the climate of Washington, D.C., will become unlike anything experienced in recent history by 2047. Anchorage, Alaska, has until 2071, while Manokwari, Indonesia, will be outside historical bounds in just seven years, by 2020.
Leaving 'normal' far behind
Of course, putting a date on exactly when a given area will enter into a new climate is a pretty difficult task, noted several scientists familiar with the research.
Yet while the precise dates might be contested, scientist Eric Post, who directs the Polar Center at Pennsylvania State University, called the analysis "striking for the breadth, depth and meticulousness of the modeling. It clearly represents an exceptional undertaking in climate change science, with real potential to advance the field."
"At the very least, the Mora paper will, I believe, kick-start a discussion of when we can expect prevailing climatic conditions to have departed so far from 'normal' that we can say we're now living under novel climatic conditions," added Post.
Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, said the paper seems "quite useful." He did, however, express some concern about the conclusions on the tropics, saying that a big source of temperature variability in the tropics is the El Niño Southern Oscillation, and many models do not include that climate phenomenon.
If world governments do decide to reduce emissions, the date when climates will change will be delayed, but the shift will still occur, said Mora.
Mora and his co-authors said the analysis demonstrates the clear need to take action on curbing greenhouse gas emissions. Taking action to reduce emissions could push back the average date of a new climate regime from 2047 to 2069, according to their analysis.
"The solutions lie in international cooperation and this happening at the highest level," said Longman.