19 October 2014

The Important Warming Isn't Global

Yes, the world is overheating. Yes, that tends to cause climate change in ways that go beyond hotter afternoons--say, the widening of the tropics, the desertification of the Mediterranean, and more extreme weather.

One would think (irony alert) that the best way to summarize the climate threat is to average the near-surface temperature 1) over day and night, 2) over all four seasons, and 3) over both ocean surface and land surface.

There is fifty years of tradition in producing the annual number this very way. This globally averaged annual surface temperature, once we subtract what it was back in the good old days, is known as "global warming."

"Global" warming is global only in the sense of including both ocean surface and land surface, not in the sense of incorporating everything. It leaves out the air blanket above your head and the ocean depths, both major places where the excess heat has been stashed. Global is just what you see on a globe—and thus a somewhat misleading use of the term.

Since there is twice as much ocean surface as there is land, the land warming only counts half as much as the ocean surface warming when calculating "global" warming. Most of us live on land and so this global average number underestimates what we experience.

We now have a half-century of experience summarizing the growing climate problem via this unnatural number. Logical as it seemed fifty years ago when introduced to track the planet's heat budget (and it is still very useful for that), it has turned out to only roughly track climate changes.

Furthermore, this unfortunate choice of terminology has generated endless confusion among nonscientists, eagerly exploited by promoters of "unfettered business as usual" who seek to delay climate action by prolonging our confusion.

Then there is that word "warming," another somewhat misleading term. It has sounded cozy to some people. "Warming" just doesn't imply an excess in the way that "overheating" does.

Some have wondered, not unreasonably, how an unusually cold winter could be consistent with a general warming. Or how there could be a pause in "global warming" while the CO2 concentration keeps rising. The climate scientists have perfectly good answers to these concerns, but they tend not to be heard—in about the same way as corrections to yesterday's news articles tend not to be read.

It has become increasingly apparent over the last fifteen years that we can have a lot of climate change, and a lot of CO2 increase, without much trend in that "global warming" number. We had a similar period from 1950 (when Popular Mechanics had a nice article on global warming) until 1977. It didn't seem to be globally warming at all, despite a lot of additional CO2 accumulation from the soaring post-war emissions.

The climate system has a lot of sectors; sometimes, cooling influences can temporarily counter the extra heating of greenhouse gases. More clouds or greater ocean downwelling could do it.

Just as a map is not the terrain, an index is often not where the action is. An analogy: the Dow-Jones Industrial Average, "The DJI," tends to underestimate what is happening in tech stocks. "The market" may go nowhere while some market sectors are thriving. 

Is something like that happening to our index of what alters climate? However useful the global average overheating may be for working out the heat budget and finding hidden sinks, is there a better index for drivers of climate change than just averaging over oceans and continents?

It is certainly worth looking for one. The two major topics at all three climate science conferences that I attended in a ten-day period in September were the Arctic Amplification and, separately, the so-called "hiatus" in global warming of the last ten years. During the third meeting, one possibility occurred to me: Don't add. Subtract.

Subtracting Arctic temperature from continental land temperature would have helped understand Arctic Amplification, prolonged heat waves, and those frigid "Arctic outbreaks."
Subtracting ocean temperature from land temperature would have been a better idea than adding them. That's important because the extra heating warms the land twice as fast as it does the ocean surface. That has consequences.

So let's talk about movers of climate change rather than the usual statistical abstractions. As is well known from monsoons, hot land heats the air just above the surface, causing it to rise, which sucks in cooler moist air from offshore.

It's just like the draw of the fireplace. This "sea breeze" peaks in the late afternoon and helps sailors get home in time for supper. Suppose climate change is strengthening the sea breeze?

So rather than adding together two parts ocean to one part land, subtract ocean from land. Compare that to what it was in the good old days. Call it enhanced coastal contrast if you like.

Think of it as a rough index of what should strengthen the moist onshore winds. That's already an improvement over that weighted sum of land and ocean temperatures which, to nonscientists, mostly suggests warmer afternoons and the need for more air conditioning.

[It's not as if atmospheric scientists don't already create climate indices based on regional differences. A pre-overheating classic is the North Atlantic Oscillation ("NAO"), which subtracts the air pressure at Iceland in the north from the higher pressure at the Azores in the south. When the difference is big (NAO+), the storm tracks often take a left turn into Europe. When the pressure difference becomes less pronounced (NAO-), the Atlantic storm tracks deliver the rain to the Mediterranean rather than Europe. Once shifted, it tends to stay that way for a decade or more, enabling predictions about winter storm tracks.]
Subtraction's focus on stronger onshore winds also helps us think in terms of rainfall, or its lack. But a stronger wind need not follow the customary path and may well deliver its moisture somewhere else. Result? Some places will get unexpected rain, others will lack enough.

Flood here, drought there, and who knows when? This generality tells us quite a lot about what’s ahead. Even minor rearrangements in wind can produce trouble. Change the month when the rains arrive and fields that supported two crops each year may only support one, cutting food yield in half. Note that no change in the annual rainfall is required for trouble—nor does it take extreme weather, as when prolonged heat waves bake all of the water out of the topsoil and the plants collapse.

Were a new pattern of moisture delivery to stick around for decades, we might slowly adapt. But no. That’s because the watery laggard in the global warming race just keeps falling further behind the continents with the years, thanks to all of that evaporative cooling of the ocean surface, its greater heat capacity, and the way surface waters are flushed into the depths in a few places. And so the continent's winds keep changing around unpredictably. That's a recipe for even more climate chaos.

As long as “Hot-Spot Temperature” keeps increasing its lead over “Sea Surface Temperature” in this horse race to Hell, the strengthening temperature contrast across coastlines can strengthen the moisture-laden winds coming off the ocean and thus change where the rain falls (or doesn't).

Welcome to climate instability, where all bets are off when recent history doesn't help you decide what to plant where.

William H. Calvin is a professor emeritus at the University of Washington’s medical school in Seattle and the author of Global Fever: How to Treat Climate Change (University of Chicago Press, 2008). The latest version of the CO2 cleanup was a finalist in MIT's 2013 geoengineering climate contest.
 October 2014 WCalvin@UW.edu faculty.washington.edu/wcalvin

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