Why So Dry? Ocean Temperatures Alone Don't Explain Droughts

Why So Dry? Ocean temperatures alone don't explain droughts Patrick L. Barry

The western United States continues to struggle with the worst dry spell since the 1930s, and an international report on climate change predicts more and worse droughts to come (see "From Bad to Worse," in this week's issue). As scientists work to understand what triggers droughts, a new finding suggests that the causes may be more complex than many have supposed.

DUNE DATA. Beneath these grassy hills of Nebraska lie the remains of ancient sand dunes, remnants from centuries-long droughts that have stricken this area several times in the past 10,000 years. Inset: the Sand Hills region (cream color). iStockphoto; (inset) E. Roell

Researchers recently pieced together the most comprehensive history yet of drought in the Great Plains region. The record covers the 10,000 years since the end of the last ice age. This new time line shows three distinct megadroughts—periods of severe dryness lasting for centuries. Scientists often attribute drought to changes in ocean-surface temperature patterns, such as those associated with El Niños. But when the research team compared its record with estimates of historical sea-surface temperatures, only the most recent of the three dry spells matched up.

"Linking Pacific sea-surface temperatures to drought doesn't explain the drought patterns that we see," says Joseph Mason, a geographer with the research team at the University of Wisconsin–Madison. The finding suggests that other factors, such as solar intensity or global wind patterns, sometimes play a role.

To detect drought in the distant past, the scientists studied buried dunes in the Sand Hills region of Nebraska. Land covered by vegetation is protected against wind erosion, but as drought lingers, the soil becomes exposed and dry. Wind can then push sand around more easily, forming migrating dunes. So, ancient dunes are a good indicator that drought has occurred.

The scientists dated the remnants of the dunes by measuring fluorescence in the long-buried sand grains. Exposing the grains to light releases a faint flash of fluorescence that's more intense the longer the period since the sand grain last saw sunlight. The technique yields age estimates good to within 10 percent, which is better than radiocarbon dating can achieve.

From these data, the scientists found a 300-year dry period that began about 1,000 years ago, coinciding with a well-known warm episode called the Medieval Climate Anomaly. They found two other epochs of desertlike conditions that ran from 4,500 to 2,300 years ago and from 9,600 to 6,500 years ago, the team reports in the February Geology. The Medieval Climate Anomaly appears linked to changes in ocean temperatures, but the earlier two droughts don't.

"It's a very nice piece of work," comments Daniel Muhs, a research geologist at the U.S. Geological Survey in Denver. The causes of droughts "may be a lot more complicated than we thought," he says. "The next step is for the atmospheric-science community to look at something like this and say, 'OK, maybe there are other mechanisms involved.'"

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"From these data, the scientists found a 300-year dry period that began about 1,000 years ago, coinciding with a well-known warm episode called the Medieval Climate Anomaly."

Nothing "anomalous" about it, they just don't have the understanding to explain it.

The "anomaly" they are referring to was between 1000 A.D. and 1300 A.D., when average global temperatures were about 3 degrees centigrade WARMER THAN THE AVERAGE TEMPERATURES ARE TODAY. It came on about as suddenly (over about a century) as today's "global warming" trend is doing (and, my goodness, there was not greenhouse-gas emitting SUV's back then either!!!!).

That "3 degrees centigrade" is also about as warm as they say the current "global warming" will achieve.

So, the earth can suddenly have temperatures "3 degrees centigrade warmer than today, on its own, lasting between 1000 A.D. and 1300 A.D, but here in 2007 A.D it can only be man made?????

Talk about arrogance and hubris!!!!

From Bad to Worse: Earth's warming to accelerate

Sid Perkins

Global warming is real and will continue, and there's strong evidence that people are to blame, an international panel of scientists has concluded. Other scientists suggest ways that people might reduce future atmosphere-warming greenhouse-gas emissions and argue that societies will have to adapt to the climate change that's yet to occur.

THE HEAT IS ON. Carbon dioxide emissions from coal burning and other human activities almost certainly contribute to global warming and will continue to do so for millennia, a new report says. Photodisc

"The evidence for warming having happened on the planet is unequivocal," says Susan Solomon, an atmospheric scientist at the National Oceanic and Atmospheric Administration in Boulder, Colo. "We can see that in rising air temperatures, we can see it in changes in snow cover in the Northern Hemisphere, we can see it in global sea rise," she says. Solomon and her colleagues on the Intergovernmental Panel on Climate Change (IPCC) released their latest assessment of recent warming trends at a press conference in Paris on Feb. 2.

The average temperatures at Earth's surface for 11 of the past 12 years rank among the dozen highest values recorded since the mid-1800s. Over the past 100 years, global average temperature has risen about 0.74°C, the IPCC researchers report. With 90 percent certainty, scientists link that increase to the rising concentrations of carbon dioxide and other heat-trapping greenhouse gases that human activities have released into Earth's atmosphere.

Carbon dioxide concentrations measured 379 parts per million (ppm) in 2005, far in excess of the fractions inferred from ice-core data representing periods going back 650,000 years. The concentration of atmospheric carbon dioxide is now growing at around 1.9 ppm per year, the largest rate of increase ever measured. Accordingly, scientists suggest in the IPCC report that over the next 20 years, the average global temperature will rise by an additional 0.4°C.

Today, coal and petroleum combustion each account for about 40 percent of global carbon dioxide emissions, says Daniel P. Schrag, a geochemist at Harvard University. The largest use of coal, burning it to generate electricity, produces about 8 billion tons of carbon dioxide each year—"more than any responsible climate change policy can accommodate," he says in the Feb. 9 Science.

Strategies to decrease carbon dioxide emissions include reducing energy use, capturing carbon dioxide at its sources and sequestering it, or expanding the application of energy sources that don't produce the gas. "It's clear that none of these is a silver bullet," says Schrag.

However, one promising technique is to lock away the gas by injecting it into seafloor sediments or by pumping it into saline aquifers or old oil and gas fields. In ongoing research, scientists at a handful of test sites sequester only about 1 million tons of carbon dioxide each year, Schrag reports.

Even the most optimistic projections of emissions limits show global greenhouse-gas concentrations rising for the foreseeable future, says Roger Pielke Jr., a policy analyst at the University of Colorado at Boulder. Future climate change is unavoidable, he and his colleagues report in the Feb. 8 Nature. Therefore, they add, adaptation to the warming yet to come will be as essential to climate policy as greenhouse-gas mitigation.

The IPCC is scheduled to address the mitigation of climate change in an April report. In May, the group will issue an assessment of the societal impact of current and future warming and is to suggest how people might best adjust to the change.

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