Effects of climate change on the water cycle

The yearly average distribution of precipitation minus evaporation. The image shows how the region around the equator is dominated by precipitation, and the subtropics are mainly dominated by evaporation.

The global water cycle describes the movement of water in its liquid, vapour and solid forms, and its storage in different reservoirs such as oceans, ice sheets, atmosphere and land surface. The water cycle is essential to life on earth and plays a large role in the global climate and the ocean circulation.

The warming of the earth is expected to cause changes in the water cycle for various reasons.^[1] A warmer atmosphere can contain more water vapor which has effects on evaporation and rainfall. Oceans play a large role as well, since they absorb 93% of the increase in heat since 1971.^[2] This has effects on the water cycle and on human society, since the ocean warming directly leads to sea level rise.^[1]

Changes in the water cycle are hard to measure. Changes in ocean salinity are important indicators of a changing water cycle. The ocean salinity patterns are observed to amplify, which is considered as the best evidence for an intensifying water cycle.

Causes of intensifying water cycle[]

The increased amount of greenhouse gases leads to a warmer atmosphere.^[1] The saturation vapor pressure of air increases with temperature, which means that warmer air can contain more water vapor. Because the air can contain more moisture, the evaporation is enhanced. As a consequence, the increased amount of water in the atmosphere leads to more intense rainfall.^[3]

This relation between temperature and saturation vapor pressure is described in the Clausius-Clapeyron equation, which states that saturation pressure should increase 7% when temperature rises with 1°C.^[4] This is visible in measurements of the tropospheric water vapor, which are provided by satellites, radiosondes and surface stations. The IPCC AR5 concludes that tropospheric water vapor has increased by 3.5% the recent 40 years, which is consistent with the observed temperature increase of 0.5 °C.^[1] It is therefore expected that the water cycle is intensifying, but more evidence is needed to say so.

Salinity evidence for changes in the water cycle[]

Essential processes of the water cycle are precipitation and evaporation. The local amount of precipitation minus evaporation (often noted as P-E) shows the local influence of the water cycle. Changes in the magnitude of P-E are often used to show changes in the water cycle.^[5] But robust conclusions about changes in the amount of precipitation and evaporation are complex.^[6] About 85% of the earth's evaporation and 78% of the precipitation happens over the ocean surface, where measurements are difficult.^[2][3] Precipitation on the one hand, only has long term accurate observation records over land surfaces where the amount of rainfall can be measured locally (called in-situ). Evaporation on the other hand, has no long time accurate observation records at all.^[2] This prohibits confident conclusions about changes since the industrial revolution. The AR5 (Fifth Assessment Report) of the IPCC creates an overview of the available literature on a topic, and labels the topic then on scientific understanding. They assign only low confidence to precipitation changes before 1951, and medium confidence after 1951, because of the scarcity of data. These changes are attributed to human influence, but only with medium confidence as well.^[1]

Oceanic salinity[]

Another method to track changes in the water cycle, is measuring the global oceanic surface salinity. Seawater consists of fresh water and salt, and the concentration of salt in seawater is called salinity. Salt does not evaporate, thus the precipitation and evaporation of freshwater influences salinity strongly. Changes in the water cycle are therefore strongly visible in surface salinity measurements, which is already acknowledged since the 1930s.^{[5] [7]}

The global pattern of the oceanic surface salinity. It can be seen how the by evaporation dominated subtropics are relatively saline. The tropics and higher latitudes are less saline. When comparing with the figure above it can be seen how the high salinity regions match the by evaporation dominated areas, and the lower salinity regions match the by precipitation dominated areas. Based on GODAS data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://www.psl.noaa.gov/data/gridded/data.godas.htm

The advantage of using surface salinity is that it is well documented in the last 50 years, for example with in-situ measurement systems as ARGO.^[2] Another advantage is that oceanic salinity is stable on very long time scales, which makes small changes due to anthropogenic forcing easier to track. The oceanic salinity is not homogeneously distributed over the globe, there are regional differences that show a clear pattern. The tropic regions are relatively fresh, since these regions are dominated by rainfall. The subtropics are more saline, since these are dominated by evaporation, these regions are also known as the 'desert latitudes'.^[2] The latitudes close to the polar regions are then again less saline, with the lowest salinity values found in these regions. This is because there is a low amount of evaporation in this region, and a high amount of fresh meltwater entering the ocean.

The long term observation records show a clear trend: the global salinity patterns are amplifying in this period.^[1][8] This means that the high saline regions have become more saline, and regions of low salinity have become less saline. The regions of high salinity are dominated by evaporation, and the increase in salinity shows that evaporation is increasing even more. The same goes for regions of low salinity that are become less saline, which indicates that precipitation is intensifying only more.^[2][9] This spatial pattern is similar to the spatial pattern of evaporation minus precipitation. The amplification of the salinity patterns is therefore indirect evidence for an intensifying water cycle.

To further investigate the relation between ocean salinity and the water cycle, models play a large role in current research. General Circulation Models (GCMs) and more recently Atmosphere-Ocean General Circulation Models (AOGCMs) simulate the global circulations and the effects of changes such as an intensifying water cycle.^[2] The outcome of multiple studies based on such models support the relationship between surface salinity changes and the amplifying precipitation minus evaporation patterns.^[2][10]

A metric to capture the difference in salinity between high and low salinity regions in the top 2000 meters of the ocean is captured in the SC2000 metric.^[5] The observed increase of this metric is 5.2% (±0.6%) from 1960 to 2017.^[5] But this trend is accelerating, as it increased 1.9% (±0.6%) from 1960 to 1990, and 3.3% (±0.4%) from 1991 to 2017.^[5] Amplification of the pattern is weaker below the surface. This is because ocean warming increases near-surface stratification, subsurface layer is still in equilibrium with the colder climate. This causes the surface amplification to be stronger than older models predicted.^[11]

Effects on oceanic circulation[]

An overview of the global thermohaline circulation. It shows how there is a northward surface flow in the Atlantic Ocean, which sinks and reverses direction in the Arctic. The freshening of the Arctic surface waters by meltwater could lead to a tipping point. This would have large effects on the strength and direction of the AMOC, with devastating consequences for nature and human society.

Near the poles, climate change induces another effect on the water cycle. The increase of the atmospheric temperatures leads to a higher rate of melt of land and sea ice. This creates a large influx of freshwater into the ocean, which lowers the salinity of the surface water locally. The thermohaline circulation in general, and the AMOC specifically, is dependent on the current high surface salinity in the Arctic. The cold and saline water has a high density (as described by the equation of state) and therefore sinks to the bottom of the ocean. On the bottom it then returns southward, this is the so-called overturning. Large flow of meltwater into the Arctic basins reduces the surface salinity and therefore this overturning effect. As described in , a tipping point can be reached when the Arctic surface salinity keeps reducing by meltwater, leading to a stop of the AMOC or a change in its direction. This would have large impact on the global climate and human societies.

This would be practically irreversible, since the system has a hysteresis loop. This means that reversing the system to the old state would require much higher salinity values than we current experience, which is the reason for the large concerns about reaching the tipping point.^{[12][13][14][15]}

References[]