Precipitation is notoriously hard to forecast accurately. The formation of precipitation is a fine balance between many factors, and small changes in any of these can influence whether it rains, where it rains, and how much rain falls. Predicting how such factors may change under global warming is even more of a challenge, particularly on regional scales. Globally there are robust projections; current research suggests that we are not just heading for a simple steady increase in global precipitation, but rather shifts towards less frequent, but heavier rainfall events1. This would result in a lose-lose situation for societies, with increases in floods in wet regions and an increase in droughts in already dry regions. Both of these can have severe consequences on human life, on the environment and economically.
Fig. 1: Relative changes in precipitation for 2090-99 relative to 1980-99. Values are multi-model averages for December to February (left) and June to August (right). White areas are where less than 66% of the models agree on the sign of the change, with stippled areas where more than 90% agree. Figure from the IPCC AR4 Summary for Policymakers (fig SPM.7)
Rain falls to the ground when cloud water droplets become heavy enough to overcome the vertical winds, or up-drafts, that otherwise keep them suspended in the cloud. How fast droplets grow is largely dependent on the relative humidity of the cloud air, that is, how much water vapour is in the air relative to a maximum (saturation) value which is determined by the air temperature. The more water-saturated the air is, the faster the cloud droplets will form and grow. While they remain suspended in the cloud, the weight of the droplets is balanced by the cloud up-drafts. The strength of these up-drafts varies with the stability of the air (i.e. the vertical temperature profile), the convergence of horizontal winds near the surface, and also with the amount of condensation occurring within the cloud. The condensation of water vapour into liquid water releases latent heat energy, warming the surrounding air and thus providing a buoyancy force to suspend the cloud droplets for longer. This positive feedback means that predicting exactly when a cloud will start to rain out is a tough job.
These interacting factors make it difficult to accurately forecast rain in the present climate, not least because the atmosphere is chaotic: tiny changes in our best guess of the initial state of the atmosphere can lead to substantial differences in any one of these factors, altering the precipitation. Predicting how these factors may change on regional scales with increasing CO2 emissions is even harder. Small variations between the future circulation changes that are projected by different climate models can result in sufficiently large differences in humidity or temperature gradients to significantly change the projection of precipitation. This goes some way to explaining why the models in the IPCC 4th assessment report2 often do not even agree on whether precipitation in a certain region will increase or decrease in the future, let alone by how much. Figure 1 shows the multi-model mean projected change in precipitation for 2090-2099 relative to 1980-1999, with stippled areas indicating where more than 90% of the models agree on the sign of the change. Similar discrepancies between models are seen for all other periods and scenarios. Greater understanding is consistently being gained on expected circulation changes, helping to make regional projections more robust; however, outside of the polar regions it is clear that a lot of model disagreement remains.
Luckily, projections of global precipitation changes are more robust. The warming from climate change means that more water vapour can be held in the atmosphere. The Clausius-Clapeyron equation relates the temperature of air to the maximum amount of water vapour it can hold before becoming fully saturated. The relationship tells us that this maximum value in the atmosphere will increase at approximately 7% per 1K of warming. Both observations and simulations suggest that relative humidity will stay more or less constant in a changing climate. Thus the absolute amount of water vapour held should increase with the saturation, or maximum, value, i.e. 7% per 1K of warming. The simplest assumption would be that global rainfall will therefore increase at this amount; however, energy is also an important constraint on the hydrological cycle. Energy availability can limit surface evaporation, which is necessary for replenishing atmospheric moisture after rainfall events. Additionally, a limitation is put on precipitation formation by the speed at which the atmosphere can remove the heat energy3 released during the condensation of cloud water vapour into rain droplets: too much warming of the cloud from latent heat will raise the maximum amount of water the air can hold. As the absolute amount of water in the atmosphere does not change, this rise in the maximum value will reduce the relative humidity, slowing rain droplet growth. These energetic constraints may explain why globally averaged precipitation is projected to increase at only around 2% per 1K of warming despite the 7% per 1K increase in atmospheric water vapour.
Interestingly, a significant body of research indicates that the intensity of the heaviest rainfall events, unlike average precipitation, is likely to increase in line with atmospheric water content4, i.e. at 7% per 1K. Combining this result with the 2% for average precipitation means that something must be changing in the distribution of rainfall events: either the intensity of light events must increase at a rate much less than the mean (or decrease) to compensate, or the frequency of heavy events must decrease so that their contribution to mean intensity lessens. This implies that global warming may induce changes in regional precipitation – regions already experiencing heavy rainfall events may have to cope with heavier downpours, whilst regions which already get little rainfall may get even less. This is the ‘rich get richer, poor get poorer’ scenario applied to global precipitation. There is already observational evidence that numerous regions of the world may be experiencing shifts to less frequent, but heavier, rain events1.
Under anthropogenic climate change it seems that Earth may well experience shifts in the distribution of precipitation towards fewer, heavier, rainfall events. This would lead to global increases in both droughts as well as floods. Furthering our understanding of this response of the hydrological cycle to a warming atmosphere is therefore an important step in understanding the consequences of climate change to societies around the globe.
References
1. Giorgi, F., E. S. Im, E. Coppola, N. S. Diffenbaugh, X. J. Gao, L. Mariotti, and Y. Shi, 2011: Higher Hydroclimatic Intensity with Global Warming. Journal of Climate, 24, 5309-5324.
2. Allan, R. P., and B. J. Soden, 2008: Atmospheric warming and the amplification of precipitation extremes. Science, 321, 1481-1484.
3. Allen, M. R., and W. J. Ingram, 2002: Constraints on future changes in climate and the hydrologic cycle. Nature, 419, 224.
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Rachel White
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