The mean annual climate of the average location on Earth will slip past the most extreme conditions experienced during the past 150 years and into new territory by between 2047 and 2069, depending on the amount of climate-warming greenhouse gases that are emitted during the next few decades, a new study found. The study, published Wednesday in the journal Nature, used a new index to show for the first time when the climate — which has been warming during the past century in response to manmade pollution and natural variability — will be radically different from average conditions during the 1860-2005 period.
The study shows that tropical areas, which contain the richest diversity of species on the planet as well as some of the poorest countries, will be among the first to see the climate exceed historical limits — in as little as a decade from now — which spells trouble for rainforest ecosystems and nations that have a limited capacity to adapt to rapid climate change.
According to the study, conducted by a team from the University of Hawaii, about 1 billion people currently live in areas where the climate will exceed historical bounds of variability by 2050. This number would rise to 5 billion people under a business-as-usual emissions scenario, which is the emissions path the world is currently on.
Map of multi-model mean results for different greenhouse gas concentration scenarios of annual mean surface temperature change in 2081– 2100. Credit: IPCC Working Group I.
Even more strikingly, the study found that the oceans, which have absorbed about half of the manmade carbon dioxide (CO2) emissions since the dawn of the industrial revolution 250 years ago, exceeded their historical bounds of pH measurements back in 2008.
In other words, the oceans are now more acidic than they have been since at least 1860.
The study “Provides a new metric of when climate change will lead to an environment that we’ve never seen before,” said author Camilo Mora, a professor at the University of Hawaii at Manoa, in an interview.
“When you look at the information there is a lot of empirical evidence suggesting that indeed we already crossed the threshold of pH variability during the last decade.”
In the water, CO2 reacts to form carbonic acid, and over the years, the ocean’s acidity has increased by more than 30 percent because so much of the excess man-made CO2 is being drawn into the water.
This increased acidity changes the balance of other carbon-species in the water, and may have far-reaching ramifications. Some marine species that use a form of carbonate to build their skeletons and shells, like corals and mollusks, may be harmed because the acid formed in the water consumes this carbonate and makes it less accessible to these organisms.
Even with aggressive cuts in greenhouse gas emissions, the study found, the projected near-surface air temperature of the average location on Earth will move beyond historical variability in about 56 years from now. A business-as-usual scenario in which emissions continue on their current upward trajectory would see an unprecedented climate occurring 20 years sooner than that, in 2047.
However, the extra 20 years that emissions cuts would buy time for making emissions cuts and could prove crucial for many species’ survival, Mora said.
Imagine you are on a highway, and you spot an obstacle in the road up ahead, Mora said. “Should you step on the gas, or hit the brake?” Hitting an obstacle at a slower speed will minimize the damage to the car and its occupants, in much the same way as hitting a climate threshold at a slower speed would reduce the ramifications for biological systems, he said. “The speed at which you face that (obstacle) is going to make a huge difference."
The study questions the way climate change is typically framed, which is by looking at the absolute value of the temperature change that is expected to occur in the coming decades. This framing often identifies the Arctic as being ground zero for the most significant and rapid climate change, and overlooks the fact that, while the Arctic has a history of bigger temperature swings, that's not the case in the tropics, where temperatures have historically remained within a narrower range. That makes it easier for a small amount of warming to make a big difference in tropical climate.
For temperature-sensitive tropical species, such as coral reefs, the speed at which climate change occurs can be a more important factor in determining how disruptive climate change will be, even though the total amount of climate change expected in the tropics will be less than in the Far North.
Chart showing the year by which the climate is projected to exceed the range of historical variability, depending on the emissions scenario. "Stabilized emissions" corresponds to a low emissions scenario. Click to enlarge the image. Credit: Alyson Kenward, Climate Central. Data from Mora et al.
“The tropics, not the poles, are going to be feeling the effects first,” Mora said.
Mora said the index he and his team developed aims to address this shortcoming of traditional climate studies. The index used the minimum and maximum temperatures from 1860-2005 to define the bounds of historical climate variability at any given location. The scientists then took projections from 39 climate models for the next century to find the year in which the future temperature will exceed the limits of historical precedents, defining that year as the year of climate departure. When the climate reaches this point, the average temperature of the coolest year at a given location will be greater than the average temperature of its hottest year for the period from 1860 to 2005.
“We analyzed every single model that has been developed so far,” Mora said. “All of that data is telling you something in common, and that is that pretty soon we’re going to be facing unprecedented climate.”
Since many tropical nations are major suppliers of food to global markets, including fish that rely on healthy coral reef ecosystems as well as many other goods and services, there may be ripple effects throughout the global economy despite the longer lag time before the climate of the industrialized nations exceed the bounds of their historical climate, Mora said.
Warming in the tropics, he said, “Will increase the prices of the things that we have to pay here.”
Given that there are considerable uncertainties about future greenhouse gas emissions as well as the precise response of the climate system to those emissions, not to mention the uncertainties inherent in computer modeling, the study should not be taken as offering precise predictions. However, the researchers found low uncertainties associated with the date ranges by which the climate would exceed previous bounds, with greater uncertainties associated with the changes likely to take place in different geographical locations.
Ed Hawkins, a climate scientist at the University of Reading who was not involved in the new study, said the findings build on previously published research about the timing of climate change impacts. “As shown by last month’s latest assessment report by the IPCC, the focus of climate science has moved from whether climate change is happening, to when and where it will be most keenly felt," he said in a statement that was emailed to reporters.
“This kind of work is particularly important as it helps to focus the minds of policy makers about when climate change will start to change our environment. It shows that in many parts of the world, climate change will start to have a major impact in our own lifetimes, not just during those of our children and grandchildren.”