When you hear the term “global warming”, do you think of the warming of air temperatures at the Earth’s surface, or the warming of the planet as a whole?
Only about 2 percent of the planet’s overall warming heats the atmosphere, so if we focus only on surface air temperatures, we miss 98 percent of the overall warming of the globe. About 90 percent of the warming of the planet is absorbed in heating the oceans. However, until the past few years, our measurements of ocean temperatures (especially of the deep oceans) were somewhat lacking. Our measurements of surface air temperatures were much more accurate, and so when people spoke of “global warming,” they tended to focus on air temperatures.
In the 1980s and 1990s when air temperatures were warming in step with the overall warming of the planet, that was fine. However, over the past decade, the warming of surface air temperatures has slowed. At the same time, the overall warming of the planet has continued, and if anything it has accelerated. This has been difficult to reconcile for those who previously focused on surface air temperatures – what do we say about “global warming” now?
The result is a spate of articles from the New York Times, Washington Post, The New Republic, and Der Spiegel, all of which get most of the facts right (including noting the warming of the oceans), but that all begin from the premise that “global warming” has slowed. It would be more accurate to say that global surface air warming has slowed, but the overall warming of the Earth’s climate has sped up.
This is the conclusion of several papers published in the past year, including studies led by Sydney Levitus of the National Oceanic and Atmospheric Administration (NOAA), Magdalena Balmaseda of the European Centre for Medium Range Weather Forecast, Virginie Guemas of the Catalán Institute of Climate Science, and myself. When the warming of the Earth’s entire climate system is considered, global warming continues to rise at a rate equivalent to about 4 Hiroshima atomic bomb detonations per second, faster over the past 15 years than the prior 15 years.
The small fraction of that warming that’s expressed by changes in surface air temperature does appear to have slowed over the past decade. Research by Masahiro Watanabe of the Japanese Atmosphere and Ocean Research Institute suggests this is mainly due to more efficient transfer of heat to the deep oceans. Consistent with model simulations led by NOAA’s Gerald Meehl, Watanabe finds that we sometimes expect “hiatus decades” to occur, when surface air temperatures don’t warm because more heat is transferred to the deep ocean layers.
Research on the causes of slowed surface air warming is of course ongoing. The question remains how much other factors have contributed to the surface warming slowdown. For example, aerosols (particulates released from volcanoes and from burning coal and diesel that cause cooling by reflecting sunlight) and low solar activity over the past decade likely played a role as well. However, Watanabe’s research suggests that these factors can’t explain most of the slowed surface warming, which his study attributes mainly to a more efficient transfer of heat to the deep oceans.
Unfortunately that isn’t a permanent solution for those of us living on the Earth’s surface. A post from a political blog for The Economist naively argued that we should just wait a couple of decades to see if surface air warming resumes. The Watanabe, Guemas, Balmaseda, and Meehl research teams all concluded that the faster warming of the oceans is only a temporary effect. Sooner or later the cycle is bound to reverse, at which point we will experience accelerated global surface air warming when the ocean heat comes back to haunt us. We can’t escape the physical reality that as long as we continue to increase the greenhouse effect, it will continue to trap more and more heat, and the planet will continue to warm.
Even if the climate is not quite as sensitive to the increased greenhouse effect as current best estimates suggest, we’re still not doing enough to reduce greenhouse gas emissions if we want to avoid dangerous and potentially catastrophic climate change. Taking a ‘wait and see’ approach for another decade or two, as recommended by the Economist political blog, would be a recipe for certain disaster. Fortunately that recommendation is at odds with the approach suggested by The Economist’s correspondents, who agree that in any case we’re not doing nearly enough to decarbonize the economy if we want to avoid dangerous climate change.
The key take-home point is that we now have better measurements of ocean and global heat accumulation. We no longer have to settle for focusing on the 2 percent of global warming represented by surface air temperatures. Consider the analogy offered by Greg Laden, that the planet is a dog and surface temperatures are his tail. In the past we only had a GPS locator on his tail. It wags around a lot, sometimes accurately representing the movement of the dog, sometimes not. Now we’ve got a second GPS locator on his body – should we continue focusing on the movement of the tail for old times’ sake, or should we shift our focus to the more representative measurements?
Ideally people will begin using the term “global warming” to refer to the planet’s overall heat accumulation. Or use the term “global heating” or “climate change” or “climate disruption.” Whatever term is chosen, we need to stop misleading people by saying that global warming has “paused.” The overall warming of the planet has not and will not pause until we stop increasing the greenhouse effect through our reliance on fossil fuels. Until we hit that ‘pause button,’ the warming will only continue to grow.
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