What is the Earth's water (hydrologic) cycle? Why is it important? How has the Earth's hydrologic cycle changed? What is the evidence for these observed changes? Are these changes natural or the result of the build-up of greenhouse gases in the atmosphere? Are these changes consistent with what climate models have predicted? What are the immediate and long-term social and economic consequences of such changes? Are we presently witnessing such consequences?

Overview

The presence and availability of water is largely what makes the Earth uniquely able to support life. Not only is water essential for life, but its presence in the atmosphere significantly amplifies the greenhouse effect. Its abundance in the oceans moderates the seasonal swing in temperatures, and its distribution over the land determines the presence and geographic extent of forests and deserts and occasionally brings floods or drought. The Earth's water (hydrologic) cycle controls the distribution of water, most importantly evaporating and distilling saltwater to create the freshwater that sustains life on land. While humans often act to beneficially control and/or correct local and even regional aspects of the water cycle such as runoff and soil moisture, the inadvertent alteration of this global hydrologic cycle by human activities will have many direct and indirect influences that significantly impact society, the environment, and ecosystems upon which our own lives and well-being depend.

Global Warming and the Earth's Water Cycle

Increases in anthropogenic greenhouse gases resulting from the burning of fossil fuels and the deforestation of forests have altered the composition of the atmosphere, resulting in an increase in the amount of heat energy trapped at or near the Earth's surface. This enhancement of the greenhouse effect is increasing surface temperatures while provoking other changes in climate as well. Both model results and observational evidence indicate that roughly 80% of the net additional heat energy trapped at the Earth's surface by the build-up of greenhouse gases is transferred back to the atmosphere through increased evaporation of water from the land and ocean, where condensation returns the additional heat to the atmosphere causing warming, while enhancing precipitation. The remaining 20% of the net additional heat from the enhanced greenhouse effect contributes directly to warming of the surface and the lower atmosphere. Both contributions lead to a general warming of the Earth's climate and to an increase in the water vapor in the atmosphere (warming increases the atmosphere's water-holding capacity), thereby further enhancing the greenhouse effect. Thus, the trapped heat energy serves to accelerate the cycling of water (as water vapor) from the surface to the atmosphere, and enhances the transfer of the water vapor back to the surface as rain and snow (condensation and precipitation). The increased availability of water vapor in the atmosphere also leads to a significant increase in the energy available to drive storms and associated weather fronts, therefore affecting rainfall rates, precipitation amounts, storm intensity, and related runoff.

The Observational Record of Changes Resulting from the Greenhouse-Enhanced Hydrologic Cycle

There is compelling observational evidence that the Earth's hydrologic cycle has intensified during the past century as global temperatures have increased. These results are consistent with climate model projections of global warming resulting from the increase in greenhouse gas concentrations. In general, the observed changes in the Earth's hydrologic cycle suggest that focusing attention mainly on the temperature effects of increased greenhouse gases (i.e., global and regional increases in the Earth's surface temperature) provides an incomplete and, in some instances, inadequate portrayal of the importance of climate change. This is so because the evidence indicates that widespread increases in the intensity of the hydrologic cycle may have more immediate and far-reaching ecological and socio-economic impacts than those due to elevated temperature alone. The observational evidence for an intensifying hydrologic cycle includes:

• A reduction in the day/night temperature range over land. Nighttime temperatures have increased at almost twice the rate of daytime temperatures since 1950 (roughly 0.9ºC versus 0.5ºC) suggesting the influence of increased evaporative cooling during the daytime (not unlike how body heat evaporates rubbing alcohol from one's skin, leaving one's body somewhat cooled in the process). Rising nighttime temperatures exacerbate heat waves and reduce the beneficial effects of frost in killing pests.
• An increase in atmospheric water vapor. It is this change that enables storms to generate more precipitation and it greatly amplifies the warming influence of greenhouse gases.
• Precipitation amounts have increased in the mid and high latitudes, often in excess of 10% since the turn of the century. This is especially important because once soils become saturated, seemingly small increases in rainfall can cause large increases in runoff, resulting in floods.
• The observed increase in precipitation has been due in large part to a disproportionate increase in heavy and extreme precipitation rates, as projected by climate models used to calculate the effects of an enhanced greenhouse effect.
• An increase in Northern Hemisphere storm intensity (outside of the tropics) has been observed over the past few decades. This increases the hazard risk along shorelines, especially as coastal populations continue to increase.

Tom holds a Masters Degree in Meteorology from the University of Wisconsin.