The scientific consensus on climate change is that atmospheric temperatures are rising and will continue to rise. Mean global temperatures are already 1˚C warmer than preindustrial times (relative to 1850–1900), predominantly due to human activity increasing the amount of greenhouse gases in the atmosphere (IPCC, 2018a). The 2020 Paris Conference of Parties (COP) agreed on the aim of a 1.5˚C cap on climate change-induced warming, although without rapidly introducing measures to reduce carbon and greenhouse gas emissions, global warming could easily go beyond this limit.
In fact, the Intergovernmental Panel on Climate Change (IPCC) warns that even a mean global temperature increase of 1.5˚C will lead to an increase in the frequency and intensity of rainfall events. But what links a warmer climate to an increase in intense rainfall events? This blog post will explain the physics behind the changes to precipitation rates in a warming climate.
A simple overview of the physics
Climate projections simultaneously warn of higher annual mean surface temperatures, higher rates of intense rainfall and more frequent intense rainfall events. The atmospheric moisture content increases with respect to a change in temperature – essentially, the warmer the atmosphere, the more water is held in the atmosphere, and therefore higher rates of precipitation can be expected.
This is explained by the Clausius-Clapeyron relationship between surface temperature and water vapour. According to the Clausius-Clapeyron relationship, atmospheric water content increases by between 6 and 7% per 1 °C. Therefore, even just an increase of 1.5°C could result in ~9% more water in the atmosphere, which could have a major impact on storm systems and subsequent rainfall.
Storm systems travelling across oceans will have an increased moisture content from water evaporated from the sea surface, forming a larger storm system and therefore more precipitation. JBA has recently discussed the risk of flooding from intensifying rainfall due to climate change and this will be explored in respect to storm systems later in this blog.
How precipitation is formed
In meteorology, precipitation can be liquid or solid water that falls from the atmosphere and reaches the Earth’s surface. Types of precipitation include rain, sleet, or snow, depending on the temperature of the atmosphere. During the water cycle (fig. 1), water evaporates from the surface into the atmosphere, and changes state from liquid to vapour. The water vapour forms cloud droplets, which join together until the heavy droplets fall from the clouds as precipitation. Several processes affect this simple view of the journey from evaporation to precipitation.
Figure 1: A diagram of the water cycle showing the connections between water masses, the atmosphere and the transpiration and condensation of water vapour.
The surface temperature – precipitation relationship in more depth
The connection between precipitation and surface temperature is defined by the Clausius-Clapeyron equations. The Clausius-Clapeyron equations calculate the energy required to cause a chemical reaction at a given pressure. In terms of precipitation, the Clausius-Clapeyron equations can be used to calculate the thermal energy required to condense water vapour into droplets when the atmospheric pressure is known.
When water droplets are evaporated into the atmosphere, they travel upwards. As the Clausius-Clapeyron relationship is dependent on atmospheric pressure, the thermal energy requirement for a phase change is lower at a lower pressure. As the water droplets travel upwards, two things happen:
- The atmospheric pressure decreases, and
- The atmospheric temperature cools (this is known as the temperature lapse rate and is typically estimated at -6.5°C per kilometre).
When the water vapour reaches an elevation where the atmospheric pressure and temperature satisfy the Clausius-Clapeyron relationship, the water vapour condenses into cloud droplets.
Impacts of a warming climate on the surface temperature - precipitation relationship
The release of carbon dioxide, and other greenhouse gases, into the atmosphere by humans has already led to climate change in the form of atmospheric warming. Long-term measurements show that the atmosphere has already warmed by 1°C since 1900. IPCC projections suggest that additional warming is inevitable, and attempts are being made to keep global atmospheric warming to under 1.5°C. Although, as previously mentioned, this could still increase the frequency and intensity of rainfall (IPCC, 2018b). To understand how an increase in annual mean surface temperature will influence rainfall events, we can apply the Clausius-Clapeyron relationship in a geographical context.
As the Clausius-Clapeyron equations define the relationship between vapour and pressure, they can also be used to define the saturation vapour pressure with respect to temperature. In meteorology, the saturation vapour pressure is the maximum pressure of water vapour, at a given temperature, before it condenses. Therefore, the pressure required to condense a water droplet increases exponentially with respect to a change in temperature.
This means that the Clausius-Clapeyron relationship can be used to determine the moisture content of the atmosphere. Warmer atmospheric temperatures will increase the atmospheric moisture content before condensation because the atmospheric pressure will not be affected by climate change in the same way as temperature. This results in the previously mentioned calculation that moisture content will increase by ~6.5% in the atmosphere per 1°C increase in temperature and means that atmospheric warming of 1.5°C will yield an increase in atmospheric moisture content of ~9%.
The effect on storms and precipitation
This ~9% increase has an impact on storm systems and therefore rainfall. Hurricane Harvey made landfall on the coast of Texas in August 2017. Over seven days, areas of Texas including Galveston and Houston experienced nearly 1.5 metres of rainfall.
Research published since the event suggests that the intensity of Hurricane Harvey is attributable to a combination of the storm stalling over one location and climate change. The Gulf of Mexico, the source of moisture for Hurricane Harvey, has experienced anthropogenic-induced sea-surface temperature warming of 1°C since preindustrial times (Pall et al., 2017; Trenberth et al., 2018). Comparing Hurricane Harvey’s precipitation records with an equivalent event from 1950, extreme value analysis concluded that climate change contributed to a 5-7% increase in rainfall rates covering the full region affected by the hurricane (Risser and Wehner, 2017).
With an increase in rainfall events and the wider impacts of climate change, it’s important for organisations to think about the potential risk to their business. JBA’s UK Climate Change Flood Model assesses and quantifies future flood risk in the UK under a warming climate and complements our range of global Climate Change Analytics, helping clients to understand and manage the effects of climate change on their assets and to enable long-term planning.
For more information on our climate change work, including bespoke consultancy services offered by our expert team, get in touch.
References
IPCC, 2018a: Summary for Policymakers. In: Global warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I.Gomis, E. Lonnoy, T.Maycock, M.Tignor, and T. Waterfield (eds.)].]. World Meteorological Organization, Geneva, Switzerland.
IPCC, 2018b. Impacts of 1.5ºC Global Warming on Natural and Human Systems. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I.Gomis, E. Lonnoy, T.Maycock, M.Tignor, and T. Waterfield (eds.)]. World Meteorological Organization, Geneva, Switzerland.
Pall, P., Patricola, C.M., Wehner, M.F., Stone, D.A., Paciorek, C.J., Collins, W.D. 2017. Diagnosing conditional anthropogenic contributions to heavy Colorado rainfall in September 2013. Weather and Climate Extremes, 17, pp 1-6.
Risser, M.D., Wehner, MF. 2017. Attributable human-induced changes in the likelihood and magnitude of the observed extreme precipitation during Hurricane Harvey. Geophysical Research Letters¸ 44(24), doi: 10.1002/2017GL075888.
Trenberth, K.E., Cheng, L., Jacobs, P., Zhang, Y., Fasullo, J. 2018. Hurricane Harvey links to ocean heat content and climate change adaptation. Earth’s Future, 6(5), doi: 10.1029/2018EF000825
