Scientists have long projected that areas north and south of the tropics will grow drier in a warming world –- from the Middle East through the European Riviera to the American Southwest, from sub-Saharan Africa to parts of Australia.

Ocean areas where plankton do not thrive, shown in black, have been expanding recently, according to new satellite studies. (NOAA)

These regions are too far from the equator to benefit from the moist columns of heated air that result in steamy afternoon downpours. And the additional precipitation foreseen as more water evaporates from the seas is mostly expected to fall at higher latitudes. Essentially, a lot of climate scientists say, these regions may start to feel more like deserts under the influence of global warming.

Now scientists have measured a rapid recent expansion of desert-like barrenness in the subtropical oceans –- in places where surface waters have also been steadily warming. There could be a link to human-driven climate change, but it’s too soon to tell, the scientists said.

[UPDATED below, 3/6, 1 p..m]

As I’ve written in a brief piece for Thursday’s paper, relatively plankton-free (and thus fish-free) stretches of the Atlantic and Pacific Oceans around the tropics that typically cover about 20 percent of the global ocean surface have expanded about 15 percent since 1998, according to the new study. The change was measured using the orbiting SeaWiFS instrument, which can measure the abundance of plankton by tracking color differences in sea water.

The drop in productivity could be related to the hotter surface waters, or changes in winds, the researchers said. Either factor could prevent deeper, nutrient-rich water from rising to nourish plankton and, indirectly, other marine life.

The authors of the study said the change could be temporary, given the short span of observations, but it matches a slight but steady warming trend in the affected ocean regions and also matches a pattern scientists have predicted would occur under human-caused global warming. The researchers, from the National Oceanic and Atmospheric Administration and University of Hawaii, said they have measured similar changes in the Indian Ocean, but with a less measurable trend.

The sea changes could be related to another shift in regions around the equator: A review of several decades of tropical and subtropical atmospheric measurements, published in the inaugural edition of the journal Nature Geoscience, found the tropics are, in essence, expanding, and doing so more rapidly than climate simulations projected (which could mean either that the models are overly conservative or nature is more complicated than anticipated).

For now, there’s no way to link the atmospheric and oceanic changes, said Jeffrey J. Polovina, an oceanographer with the National Marine Fisheries Service in Honolulu and an author of the analysis, which has been published in Geophysical Research Letters. “The fact that we are seeing an expansion of the ocean’s least productive areas as the subtropical gyres warm is consistent with our understanding of the impact of global warming,” he said. “But with a nine-year time series, it is difficult to rule out decadal variation.”

Yet again, society is left with building [UPDATE. “hints” way better than “evidence” here, as per helpful comment below] hints of looming troubles from human-caused global warming. But, as an army of experts has been saying for two decades, if people wait around for perfect evidence, it will be that much harder to start trying to blunt the trend.

[UPDATE 3/6, 1 p.m.:] Isaac Held, a climate modeler at the Geophysical Fluid Dynamics Laboratory in Princeton, N.J., responded today with some caution about seeking relationships between the ocean and atmospheric changes around the tropics, and also drawing conclusions about their relationship to global warming. His note is pasted below in full.

He also suggests it’d be valuable for me to do a story more broadly examining the tension over interpreting dramatic shortish-term events in a realm still beset by a paucity of data, and in some cases, even hypotheses. I wrote back that my to-do list does include a look at the notion of tipping points in this context. That expression has become almost boilerplate of late in discussions of climate projections.

But it appears hard to justify, according to several scientists I’ve talked to of late, including Ken Caldeira at the Carnegie Institution. Thresholds toward big abrupt changes are likely out there, but which transitions are truly like a “one-way door” to a new state?

Efforts to identify real nonlinear thresholds seem mired in uncertainty and incomplete characterization of things like ice-sheet dynamics. Consider this an open call to scientists who have views on the merits, and drawbacks, of this description of the climate system and related arenas.

Here’s Dr. Held’s note on subtropical changes in the oceans and air:

I do not see a very close connection between the observations of a poleward expansion of the atmospheric circulation and the (primarily eastward) expansion of low-productivity waters described in Polovina et al. Jorge Sarmiento and his group here at Princeton have discussed

this paper and Jorge can give you more details on reservations that he has…, but one bottom line is that 9 years is obviously a very short time for detection of global warming trends. In

models at least, this kind of response would be most directly related to increases in stratification due to surface warming, as I understand it, and not directly to the kind of change in atmospheric circulation discussed in Dian’s paper.

So, while it is tempting to bring together the expansion of atmospheric and oceanic “deserts”, I would advise keeping them as separate issues — although I can see how this might be very difficult. I guess I would feel better about this kind of juxtaposition if the expansion of

the low-productivity zones described here were primarily poleward.

There is another important angle to the ocean productivity issue that you might consider — there is a very real danger that the satellites that provide climate-research quality observations of this kind will not be available in the near future due to limited budgets and other priorities. Jorge and others here can provide you with more details if you are interested.

I think it would be great if you did focus on the atmospheric poleward expansion and some of its implications in another article. This is a very active area of research at present, and its pretty complicated. On the one hand, models do pretty well at capturing the poleward expansion

in the Southern hemisphere but only if they include the effects of the Antarctic ozone hole! (I recall that we discussed the Australian drought as a nice focus for this question once.) In the Northern Hemisphere, the trends seem to be much larger than what our models are suggesting, a familiar theme once again.

Pursuing this last point, it is clear that in the coming few decades we are going to be continually confronted with observations of trends or events of just this type — relatively short records; much larger magnitudes than our models suggest — raising the question of whether, on the

one hand, models/theories are underestimating the rapidity of the response or missing something fundamental or, conversely, whether it is internal variability. You can make your own list, I am sure — the retreat of the Arctic ice last summer, Greenland melt, trends in Atlantic

hurricanes over the past 20 years, etc. Our models are conservative in the sense that they invariably predict slow steady trends, typically very difficult to observe over, say, a 9-year period, so this question comes up automatically for just about anything that we see that is

big and fast. There might be value in a more generic piece focusing on this kind of tension, which is going to be with us for some time — the slow steady responses predicted by current models, the concern that some observations of faster and bigger changes might actually be the greenhouse gas-forced signal and not just internal variability, the patience required before new observations and better theories/models sort things out.

Tony Broccoli, a longtime colleague of Dr. Held’s who’s now at Rutgers, wrote this:

I’d like to expand on Isaac’s comment about the challenges in interpreting relatively short series of observations. When comparing observations with model projections, it is likely that we will frequently see apparent trends that are more rapid than projected. One reason is that we have become used to seeing climate projections depicting slow but steady trends; however, this appearance comes from averaging the results from many simulations. While this ensemble averaging succeeds in identifying the robust climate change signal, it does so by averaging away the random fluctuations in climate. Even if our models are correct, the real climate system will

include both the robust response and the fluctuations, and thus occasionally exhibit larger trends when the fluctuations and the trends are both in the same direction.

The dangers of comparing smooth ensemble model projections with real data can work both ways, of course. Just as observations can appear to show a trend that is more rapid than projected, as is perhaps the case with recent trends in Arctic sea ice extent, they can also appear to show the opposite type of discrepancy. This is precisely the problem with comparing short observational time series and model projections that was exemplified in the January 10 blog post (”A Spot Check on Global Warming”) by your colleague John Tierney. The authors of RealClimate have also written about this problem.

Our interest in understanding the many dimensions of human-induced climate change will lead to many studies that illuminate novel aspects of observed climate change, such as the work by Polovina et al. and Seidel et al. As intriguing as such results will be, an appropriate caution regarding short time series remains in order. Is the climate system responding more rapidly

that we expect? We may have to live with some ambiguity. I second Isaac’s call for patience.

Tony

Anthony J. Broccoli

Department of Environmental Sciences

School of Environmental and Biological Sciences

Rutgers University

Jeff Severinghaus of the University of California, San Diego (Scripps), weighed in as well:

Dear Andy,

I applaud your interest in this topic, and I second Isaac’s suggestion that you write a more generic piece on this tension. One way to frame it for a public audience might be to say

something like,

“Dear Abby: My doctor advised me to stop smoking ten years ago, because I am at elevated risk for heart failure, but I ignored her advice. For the past month I have noticed an irregular slowing of my heart rate while resting. At a recent office visit, I asked my doctor, is this a sign of an impending heart attack?

She answered, ‘of course, the medically accurate answer is, maybe yes, maybe not. Your heart rate usually slows when you are resting. But I am much more concerned by the gradual upward trend in your blood pressure over the past 7 years.’

Abby, is my smoking habit going to give me a heart attack?”

One thing that you could stress in such a generic piece is that focusing too much on telltale symptoms is kind of a red herring, and distracts the public from the issue at hand. The reason for scientific concern over global warming rests on basic physics - infrared absorption - and robust measurements of atmospheric gas concentrations. Basic physics doesn’t make “news”, as you have so eloquently written in the past.

Jeff

Natalie Mahowald of Cornell had these thoughts:

I agree with what has been said here, but want to make a couple small points:

Siedel et al., are focused on atmospheric signs of a shift due to GHG.

The terrestrial ecosystems are really driven by changes in soil moisture. Soil moisture is driven by precipitation (predominately) and temperature (and perhaps winds). Dai et al., 2004 (Journal of Hydrometeorology, 1117-1130) argues that global mean Palmer Drought Severity index changes are driven by not just shifts in precipitation, but ALSO increase in the mean temperature. So global warming by itself should reduce water available to plants.

On the other hand, on the terrestrial ecosystem side, some of this decrease in soil moisture could be offset by CO2 fertilization–the extent to which is not known (and very controversial–I’ve had people in both camps tell me that it is KNOWN what the right answer is–and given me a completely different answers!). Deserts in on FACE study (Smith et al., 2000) experience higher productivity in a CO2 richer environment (although this includes a shift to invasive species!). I show in Mahowald (2007–GRL) that we could be experiencing an increase or decrease in desert area, depending on the strength of the CO2 fertilization–we don’t have the data to discern which is true.

Take care,

Natalie

Natalie Mahowald, Department of Earth and Atmospheric Sciences,

Cornell University

dotearth.blogs.nytimes.com/2008/03/06/expanding-deserts-by-land-and-sea/index.html