The Water Cycle and Climate Change

The Water Cycle and Climate Change

Water is always on the move. Rain falling today may have been water in a distant ocean days before. And the water you see in a river or stream may have been snow on a high mountaintop. Water is in the atmosphere, on the land, in the ocean, and underground. It moves from place to place through the water cycle, which is changing as climate changes. Below are examples of some changes that are happening as global temperatures rise. 

Climate change affects evaporation and precipitation.

Climate change is likely causing parts of the water cycle to speed up as warming global temperatures increase the rate of evaporation worldwide. More evaporation is causing more precipitation, on average. We are already seeing impacts of higher evaporation and precipitation rates, and the impacts are expected to increase over this century as climate warms.

Higher evaporation and precipitation rates are not evenly distributed around the world. Some areas may experience heavier than normal precipitation, and other areas may become prone to droughts, as the traditional locations of rain belts and deserts shift in response to a changing climate. Some climate models predict that coastal regions will become wetter and the middle of continents will become drier. Also, some models forecast more evaporation and rainfall over oceans, but not necessarily over land. 

As some areas become more prone to droughts, they are at risk of wildfires and crop failures in the dry conditions. Other areas, where heavy rainfall is becoming more common, are more prone to damaging floods.

Mudcracks form during droughts when the ground dries out and moisture evaporates.
Credit: NOAA

There’s evidence that storms are changing their character as climate changes. With more evaporation, there is more water in the air so storms can produce more rainfall.  In the case of hurricanes and other tropical storms, warmer ocean surface waters can intensify such storms, leading to more hazardous conditions as these storms make landfall. Although there may not be more tropical storms worldwide in the future, it’s likely that there will be more powerful and destructive storms.

Warmer temperatures associated with climate change and increased carbon dioxide levels may speed plant growth in regions with ample moisture and nutrients. This could lead to increased transpiration, the release of water vapor into the air by plants as a result of photosynthesis.

Clouds affect the climate and climate affects the clouds. 

Currently, the combined effect of all clouds is one of net cooling, meaning that clouds are dampening the rate of climate warming. But scientists are looking into whether clouds will have the same effect on climate as the Earth continues to warm. If the proportion of different cloud types changes, it could affect the rate of climate change because different types of clouds have different impacts on the Earth's climate. While some types of clouds help to warm the Earth, others help to cool it (as described below). This is an area of ongoing research.

Worldwide, sea level is rising because of climate change. 

Today, sea level is 10- 20 cm (4-8 in) higher than it was a century ago because of climate change. Over the 21st Century, sea level is expected to rise 30 cm (12 in) if the amount of climate change is minimized. Sea level will rise up to one meter (about three feet) if climate change is not controlled.

Global sea level rise since 1990 as measured from satellites

Credit: NASA

There are two ways that our warming climate is causing sea level rise.

First, water from melting glaciers and ice sheets flows down rivers and is added to the ocean. With less ice trapped on land in glaciers and ice sheets, there is more water in the ocean, and sea level is higher. Melting ice that is already in the ocean, like sea ice, does not cause sea level rise.

Second, ocean water expands as it warms, increasing its volume, so the water in the ocean takes up more space and sea level is higher. Since 1955, more than 90% of the excess heat held in the atmosphere by heat-trapping gases has made its way into the ocean. If this didn’t happen, climate warming would be much more dramatic. But because the heat is added to the ocean, and because ocean water expands with heat, sea level rises, flooding coasts. Also, marine life that are sensitive to changes in temperature struggle to survive.

Thermal expansion and melting ice each contributed about half of the recent sea level rise, though there is some uncertainty in the exact magnitude of the contribution from each source. Thermal expansion of seawater is predicted to account for about 75% of future sea level rise according to Earth system models.

Melting ice also causes more warming. 

Almost all of the sunlight that hits snow and ice is reflected back out to space. As climate warms and snow and ice melt, the ocean and land that were underneath are exposed. Because these surfaces are darker, they absorb more sunlight and warm more than while and light-colored surfaces. The heat is then released into the air above, which causes more warming.


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