September 21, 2007

Soil Moisture Matters

For decades, climate scientists have run numerical experiments to predict the climate response to the buildup of greenhouse gases and the answer consistently falls on the side of warming on the global scale. The climate models have become more sophisticated by orders of magnitude over the past 40 years, and the prediction of warming given increased concentrations of greenhouse gases remains as the central pillar in the global warming issue. The fact that the Earth has warmed over the past three decades makes it very easy to claim that greenhouse gases are increasing, models predict warming, the Earth’s temperature is increasing, and therefore, the science debate on the issue is over.

We at World Climate Report have confessed repeatedly that some part of the observed warming over the past three decades is very likely related to the ongoing buildup of greenhouse gases. However, thousands (actually, millions on the internet) of presentations on global warming feature claims that climate models are predicting more floods, more droughts, more hurricanes, more glacial melting, more sea level rise, more species extinctions, more … anything you can possibly name (assuming what you name is potentially catastrophic). Our essays repeatedly show that (a) models really don’t make such predictions, (b) models are not capable of such predictions, and/or (c) there is no evidence that such predictions are supported by observational data.

Two articles have appeared recently that combine independently to tell another interesting tale about models and the ability of models to predict future climate patterns. The first of these two articles is by a group of scientists at the Institute for Atmospheric and Climate Science in Zurich, Switzerland who basically explored the role of soil moisture levels in contributing to heat waves in Europe. The Fischer et al. group state “As regards climate change, model simulations indicate that extraordinary hot summers over Europe and other mid-latitudinal regions will become more frequent, more intense and longer lasting in the future, partly associated with an increase in interannual temperature variability.” No doubt, if temperatures go up and variability increases, heat waves would definitely become more frequent.

Heat waves are easier to produce when regional soils are dry and when the Sun’s energy is not evaporating water but rather heating the air. The Swiss team conducted a series of numerical experiments regarding European heat waves in 1976, 1994, 2003, and 2005 using models that had varying and fixed soil moisture levels. They concluded that “During all simulated events soil moisture-temperature interactions increase the heat wave duration and account for typically 50–80% of the number of hot summer days.” They further conclude “The regional climate model experiments reveal a major contribution of land-atmosphere interactions to the spatial and temporal extent of all four heat waves.” The bottom line is that climate models must be able to simulate regional soil moisture levels to have any hope of accurately simulating heat waves and droughts into the future.

The second article sheds some very interesting light on the issue of climate models and soil moisture levels. The article was produced by scientists at Rutgers University and the same climate group in Switzerland involved in the first article. The Li et al. team explored how well climate models have simulated soil moisture levels in the mid-latitudes of the Northern Hemisphere using both model outputs and observational data. They note that “A general drying in midlatitude summer was reported in several model simulations, which, if correct, poses a great threat to future food security.” Not only is food security threatened, by a general drying, based on the Fischer et al. work could exacerbate potentially deadly heat waves. Things are looking really bad for our future based on soil moisture trends predicted by climate models—right?

Li et al. gathered soil moisture data for the Ukraine and Russia, and in case you haven’t noticed, these two countries cover a substantial amount of the landmass of the mid-latitudes of the Northern Hemisphere. And they found, instead of drying, a general trend towards wetter conditions. Then they compared the observational data to 25 different model simulations used by the United Nation’s Intergovernmental Panel on Climate Change (IPCC). Despite climate models predicting a substantial drying, they report that “Observations from both regions show increases in summer for the period from 1958–1999 that were larger than most trends in the model simulations. Only two out of 25 model realizations show trends comparable to those of observations.” Ouch—they looked at 25 of the highly-relied upon model outputs from the famed IPCC, and … the models came up a bit short, to say the least.


Figure 1. Summer (JJA) soil moisture changes for Ukraine and Russia for the period of 1958–1999 from observations and model ensembles, expressed as anomalies relative to the 1961–1999 means. Right panel shows linear trends. Thick solid lines are estimated trends for observations and 90% confidence intervals are dashed lines. Trends from individual model realizations are plotted as asterisks in light gray. Error bar represents the standard deviation of the trends from 25 realizations (from Li et al., 2007).

OK – we are sure you get the message. Models must realistically simulate soil moisture changes to have much hope of accurately forecasting droughts or heat waves. When put to the test of comparing soil moisture predictions to soil moisture observations, they fail miserably.

Enough said from the water front!

References:

Fischer, E. M., S. I. Seneviratne, D. Lüthi, and C. Schär. (2007), Contribution of land-atmosphere coupling to recent European summer heat waves, Geophysical Research Letters, 34, L06707, doi:10.1029/2006GL029068.

Li, H., A. Robock, and M. Wild (2007), Evaluation of Intergovernmental Panel on Climate Change Fourth Assessment soil moisture simulations for the second half of the twentieth century, Journal of Geophysical Research, 112, D06106, doi:10.1029/2006JD007455.




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