In global warming, all research is local

Ozone is a tricky gas. In the stratosphere, it shields us from the sun's ultraviolet radiation. But it's an obnoxious pollutant at ground level. Now research tells us that it also impairs the ability of plants to absorb the global-warming gas carbon dioxide (CO2). That dampens the hopeful assumption that plants will take enough CO2 out of the air to partly offset the emission of that gas from burning fossil fuels.

It also highlights a significant weakness in scientists' ability to assess global-warming climate change. Various types of air pollution can change the way planetwide average warming affects local or regional climates.

Thus local circumstances can reduce, or even reverse, expectations based solely on global studies. The new ozone study, led by Stephen Sitch with England's Hadley Center for Climate Change, illustrates this.

Ozone pollution reduces plant CO2 uptake by as much as one-third, depending on local ozone concentrations. These vary widely around the world.

To take proper account of the ozone effect in climate simulations, scientists need more detailed knowledge of local ozone concentrations throughout the globe and how these change over time. The Hadley study, published online by Nature last month, is a first step in gaining such knowledge.

New research on sooty, so-called "brown cloud" pollution makes a similar point.

Veerabhadran Ramanathan at Scripps Institution of Oceanography in La Jolla, Calif., leads an ongoing study of these clouds over southern Asia and the Indian Ocean. His progress report in Nature earlier this month showed that the clouds trapped enough heat to double global warming over the Himalayan glacier region. This is speeding up the melting of those glaciers – Earth's third-largest frozen fresh water reservoir.

News reports of the study emphasized the threat this poses to regional water supplies but tended to miss a scientifically important point. The brown clouds float at heights of 1 to 3 kilometers (1/2 to 2 miles). Past global studies suggest the clouds cool the surface by blocking some of the incoming solar radiation. But, depending on regional geography, they can instead heat things up.

Dr. Ramanathan notes: "The conventional thinking is that brown clouds have masked as much as 50 percent of the global warming by greenhouse gases through so-called global dimming. While this is true globally, this study reveals that over southern and eastern Asia, the soot particles in the brown clouds are intensifying the atmosphere warming trend caused by greenhouse gases by as much as 50 percent." Here again, regional pollution contradicts an expectation based on global analyses.

Even CO2 itself defies expectations. Since plants feed on CO2, higher concentration of the gas at ground level invigorates plant growth. Climate forecasters have taken comfort in the expectation that more bountiful harvests would partly compensate for global warming's downside. However, research with crops grown in a CO2-enriched atmosphere suggests that they lose nutritional value. Detailed knowledge of how local CO2 concentrations affect crop food quality is needed.

Two weeks ago, Nature published an overview of some of the research that warns it is too early to cheer enhanced crop growth. Like the sea that is full of water we can't drink, those CO2-enhanced harvests may produce crops less fit to eat.