Input: more leafy vegetation. Output: less rainfall?
plants to take up through their root systems. The atmosphere, in turn, was sponging up most of its moisture from bare ground instead of from lush vegetation.
To tackle the problem, Lawrence and Chase borrowed a simpler hydrology scheme from a less complex biosphere model. Programming this simpler hydrology into the Community Land Model, they were able to simulate global temperature and precipitation patterns that matched observations.
“Complexity doesn’t always get you a better large-scale simulation. If you aren’t correctly representing complex interactions, you can be much better off keeping it simple,” said Lawrence. 
Peter Lawrence, a CIRES Visiting Fellow in 2004 and CIRES scientist until 2008, is now with the National Center for Atmospheric Research, working on the integration of landcover and climate.
processes. In one, the sun’s heat directly evaporates moisture from leaf surfaces, soils, and open-water sources. In the other, water is lost from plants by transpiration, a gas exchange associated with photosynthesis. Together, the two processes are called evapotranspiration.
Transpiration is an important global humidifier, contributing nearly 50 percent of all evapotranspiration worldwide, Chase said. But in the Community Land Model, transpiration was contributing just 15 percent. Evaporation from bare soils was putting three times as much water into the atmosphere.
“Water is a very strong climate modifier,” said Chase. “It impacts surface temperature, precipitation, and cloud formation. If we can’t capture fundamental hydrological processes in our climate models, we have no way to determine how human activities are affecting the climate system.”
Lawrence and Chase discovered that the model’s hydrology was based on drainage patterns typical of watersheds just a few square miles in area. Yet, a single point in a global climate model can represent several hundred square miles of Earth’s surface.
The hydrology simply wasn’t scaling to size in the model. Too much water was draining laterally, leaving little moisture for
Imagine a climate model as a black box. You put something in, you get something out. But what happens when the output is completely unexpected?
“If you don’t get the results you expect, that’s when you start to ask why,” said Peter Lawrence. Working with CIRES Fellow Tom Chase, Lawrence was comparing climate simulations from the Community Land Model – part of a select group of global models used in the Intergovernmental Panel on Climate Change’s 2007 climate change report – against observations.
The model simulations weren’t checking out.
Despite adding more leafy vegetation to the modeled planet’s land surface, Lawrence and Chase found the simulated climate consistently produced less rainfall. “Imagine adding more tropical rainforest to the planet and getting a drier, more desert-like climate,” said Chase. “It just didn’t make sense.”
Their hunch? There was a snag in the model’s water cycle.
Water on land eventually makes its way into the atmosphere through two
