Climate feedbacks: Rain and moisture
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A shift in rainfall patterns in a particular area can affect the local biosphere and ecosystems. If rainfall decreases, this may disturb the growth cycle of trees. Because trees release moisture into the air through their leaves – in a process called transpiration – fewer or smaller trees may result in less moisture in the air, leading to even less rainfall and in turn even fewer trees. This positive feedback cycle may even lead to ‘tipping points’ beyond which a forest ecosystem may be so damaged that it doesn’t grow back even if the climate returns to its original state.
Plants and trees have pores on their leaves – stomata – which enable the exchange of gases such as carbon dioxide between the leaves and the surrounding air. When stomata open, water droplets are released and usually evaporate from the leaf’s surface. This process is known as transpiration and provides an important source of moisture for cloud formation and rain. In a drought, vegetation is often damaged through lack of water. This in turn can reduce transpiration, since plant damage leads to fewer healthy leaves. Decreased transpiration affects the local water cycle by reducing the amount of available atmospheric moisture. Conversely, increased rainfall can boost plant productivity, leading to increased transpiration and more rain.
Moisture released from plants and trees through transpiration is a significant source of atmospheric moisture and plays an important part in the water cycle. The Amazon is the largest rainforest in the world and home to the largest diversity of plant species anywhere on Earth. Some climate model simulations suggest that changes in vegetation in the Amazon, such as those resulting from deforestation, could affect rainfall patterns in Texas and the Gulf of Mexico. Temperature and rainfall also affect tree growth, so any changes in local or regional climate conditions can in turn affect Amazon ecosystems. This is one example of the many complex interactions between different components of the Earth system.
The level of rainfall a forest receives affects the amount of water present in the leaves, branches and trunks of the trees, as well as the moisture in the dead leaves and other ‘forest litter’ on the ground. The drier a forest is, the higher the risk of forest fires, which release smoke containing tiny particles of soot and other aerosols. Scientists studying the effects of this smoke have found that it can suppress rainfall from clouds in the surrounding area. This is because the presence of so many aerosol particles causes many smaller cloud droplets to form, which may never grow large enough to fall as rain. As a result, if a decrease in rainfall over a forest leads to an increase in forest fires and smoke, this in turn can cause a further drop in rainfall.
Scientists use computer simulations to help them understand the behaviour of the climate system. These climate models divide the globe into grid boxes about 100 km across and represent the climate conditions in each box numerically through quantities such as air temperature, cloud cover and precipitation. Because rainfall occurs on a spatial scale smaller than a typical model grid box, scientists rely on atmospheric measurements to instruct the models how to simulate rainfall behaviour. For example, by comparing cloud and rainfall data with measurements of other atmospheric conditions taken at the same time, scientists can gain an understanding of the factors that cause a cloud to produce rain. These data can be used to formulate rules governing cloud and precipitation behaviour in climate model simulations.
Scientists take surface-based rainfall measurements using instruments known as rain gauges, which collect liquid precipitation. The daily rainfall amounts are recorded either manually or electronically. Rain gauge measurements date back over 100 years and in recent decades scientists have built up a global network of several thousand measuring stations. More recently, instruments carried on board scientific aircraft or satellites have also been used to gather precipitation data in the atmosphere or from space. Scientists analyse both short- and long-term rainfall trends in order to gain a better understanding of the complex set of interactions governing the water cycle.
Climate models are computer simulations that incorporate the different components of the climate system and the interactions between them. This includes the atmosphere, oceans, ice sheets and land, as well as forests and other vegetation. The models simulate the effects of plants and trees on the water cycle through processes such as transpiration. Simulations also account for vegetation’s low albedo – the relatively dark surfaces of leaves and branches absorb almost all of the sunlight hitting them, keeping the local area warmer than it would otherwise be. The more complex models also incorporate biological processes and feedbacks, such as the response of plant photosynthesis to changes in sunlight.