The moistening of planet Earth

The moistening of planet Earth
August 1, 2000

General circulation models don’t even scratch the surface when it comes to simulating climate. And we mean that literally.

Most models fail to properly account for moisture in the top portions of the soil. But as vegetation’s main moisture source, soil moisture obviously is related to plant growth and transpiration, the amount of moisture in the air, surface runoff rates, and so on--all critical factors if you wish to accurately model our planet’s climate.

Rutgers University Professor Alan Robock and a slew of international colleagues are in the process of assembling a global data set of soil moisture measurements. Their recent report of these trends in the latest issue of the Bulletin of the American Meteorological Society is yet another harbinger of the greener era to come.

While their data bank is not globally complete at this point (since soil moisture has not been routinely measured at many sites), it does contain good observations across much of Eurasia as well as Illinois and Iowa, with more than 600 stations available at present. Data on summer (growing season) trends in the regions with the longest records show that, in most cases, soil moisture has been increasing; in none is it decreasing.

Those of you who have been following the global warming issue will hardly be surprised that many GCMs predict that summertime drying will accompany CO2 increases.

Not only do these stations all show increasing soil moisture levels (as well as increased precipitation), but they also are getting warmer. So why, with all this additional heat, haven’t soil moisture levels dropped? Probably because most of the warming has been at night, leaving daytime evaporation mostly unaffected. And increasing precipitation, which may or may not be linked to higher temperatures, is nothing but good news for plants thriving in the robust enhanced-CO2 environment.

Robock and colleagues also compared summer soil moisture trends as predicted by the Geophysical Fluid Dynamics Laboratory’s GCM over the region of interest. Robock notes that:

“although this model predicts summer desiccation in the next century, it does not in general reproduce the observed upward trends in soil moisture very well. . . . [T]he model also does not simulate the observed upward trend in precipitation in most locations.”

With this additional good news for plants (and those who eat them), we seem to be entering an era in the global warming debate when it’s becoming increasingly difficult to figure out why these slight increases in global temperatures are any cause for alarm.


Reference

Robock, A. et al., 2000. The Global Soil Moisture Data Bank. Bulletin of the American Meteorological Society, 81, 1281-1299.