October 10, 2006

Raining on Greenhouse Predictions

Filed under: Precipitation

Every school child in our country has heard that burning fossil fuels emits carbon dioxide (CO2) into the atmosphere and that this extra CO2 will act like an extra blanket covering the Earth thereby causing temperatures to increase. The story is a bit more complex, and could go something like this: burning fossil fuels indeed emits CO2 into the atmosphere, and the CO2 alone should cause the temperature to rise a bit if all else is held equal. As the temperature creeps upward, evaporation will increase worldwide, and the atmosphere will increase its water vapor content as well. Water vapor is a powerful greenhouse gas, and the increase water vapor is far more important to the resulting temperature rise than the extra molecules of CO2.

The climate models all include this water vapor feedback, otherwise, the projected temperature rise would seem insignificant and not worthy of policy actions or funding of scientific research into global warming. To say the least, the water vapor feedback is important!

The extra water molecules floating around will ultimately fall back to the Earth in various forms of precipitation. All models predict that the planet will warm as greenhouse gas concentration increases, but they also predict precipitation will increase when averaged globally. The Intergovernmental Panel on Climate Change (IPCC) states in the Summary for Policymakers “Based on global model simulations for a wide range of scenarios, global average water vapour concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter.” In the body of the IPCC text, we learn that an ensemble of climate models predicts an average increase of approximately 4% in global precipitation over the next 75 years with a range from near 1% to near 7%. The message is loud and clear – just as all models predict an increase in global temperature, they are also predicting an increase in global precipitation.

With so much focus on detecting the temperature fingerprint, surprisingly little work has been done detecting the precipitation fingerprint. A recent article by Thomas Smith and others at the University of Maryland offers some light on the subject. The team cleverly merged rain gauge precipitation data with measurements made from six different satellite-based data sets. The satellites use infrared and microwave measurements, along with complicated computer algorithms, to make estimates of precipitation across the planet. They argue that by mixing the data from the different satellites, inhomogeneities associated with just one set of measurements are reduced. The use of the rain gauge data provides an empirical check on the satellite-based estimates.

They built a record of precipitation for 2.5º grid cells all over the world and they applied a sophisticated multivariate statistical procedure to identify modes of spatial and temporal variability in the dataset. The analysis would locate areas of the world with similar variations and/or trends in precipitation over their 1979-2004 study period. The punch line in the abstract is “Trends have spatial variations with both positive and negative values, with a global-average near zero.” Oops, that is not the right answer given predictions of the numerical climate models.

Smith et al. provide maps of where precipitation is increasing, decreasing, and staying about the same. The quantified the El Niño Southern Oscillation (ENSO) cycle and found “Most variations are associated with ENSO and have no trend.” They found that sea-surface temperatures (SSTs) have risen in the tropical Pacific and Indian Oceans, and accordingly they reported “increased tropical precipitation over the Pacific and Indian Oceans associated with local warming of the SSTs.” More water is evaporating from the ocean in this tropical region, condensing in overhead clouds, and falling right back into the ocean.

So let’s see how the models are doing. The models predict an increase in global precipitation, and none is observed. The models predict relatively large increases in precipitation in northern mid- to high latitudes and Antarctica in winter, and no increase in these areas is observed. The models do not predict much of an increase in temperature or precipitation in the tropical region of the Pacific and Indian Ocean, but that area shows the largest increase in precipitation anywhere in the world (offset by decreases in precipitation elsewhere).

It is no wonder we hear so much about the temperature “fingerprint” and so little about any precipitation “fingerprint” – that “fingerprint” simply does not exist.


Smith, T.M., Yin, X., and Gruber, A. 2006. Variations in annual global precipitation (1979-2004), based on the Global Precipitation Project 2.5º analysis. Geophysical Research Letters, 33, 2005GL025393.

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