Climate change and its impacts on the water cycle; how can it increase both droughts and heavy downpours?

If your tap ran dry and stores closed down, where would you get your freshwater? Water is essential to human life – so important, in fact, that civilizations throughout history have developed around this vital resource. In modern times, many people are fortunate enough to rely on a running tap, but for others, freshwater remains scarce. For those who always have it, water may seem like an abundant, self-replenishing resource. But in reality it is transported, transformed, and stored around the planet and atmosphere in a process called ‘the water cycle’ – with some regions receiving much more than others. Climate change can disrupt this process in a number of ways that impacts human lives, threatening sources and availability of freshwater^[1]. However, how these changes play out can be surprisingly counterintuitive, as we will later explain.

In this article, we will share scientific evidence of the impacts climate change has on Earth’s water cycle, and conclude by arming our readers with an understanding of common misinformation tactics and arguments that may otherwise be confusing and misleading.

 

Key Takeaways:

• Since the 20th century, human-caused climate change has driven observable changes in the water cycle; greenhouse gas emissions disrupt Earth’s energy balance by increasing radiative forcing and global temperatures, causing complex changes our climate including:
  • increased atmospheric moisture;
  • increased precipitation intensity; and,
  • increased global patterns of aridity (i.e., dry regions)
• Water cycle changes can negatively impact human lives by decreasing water availability, and increasing flooding through precipitation extremes (e.g., heavy rainfall).
• Some water cycle changes vary by region, but projections show future increases in precipitation nearly worldwide, and a greater total land area that will experience increased frequency and severity of drought.
• Certain changes are expected under all greenhouse gas emission scenarios (e.g., increases in precipitation extremes in all regions and increases in melting and loss of glacier mass).
• Other water cycle changes and subsequent human impacts can be mitigated with reduced greenhouse gas emissions; for example, the number of people expected to suffer from water scarcity is roughly cut in half by limiting global warming to 1.5°C instead of 2°C

 

Changes in the water cycle can negatively impact freshwater resources and human lives

 

Water is almost always in motion. It moves upwards into the atmosphere through evaporation, and downward as it cools and precipitates as rain and snow. It moves through and across land as rivers, streams, and groundwater flows. Even when it appears stagnant in ponds or lakes, or frozen in glaciers, snow, or ice sheets – over time, water is slowly escaping as a rising gas or flowing liquid. The continuous, circular process of transformation and transport of water across the planet is known as Earth’s water cycle (Figure 1).

 

Figure 1 – Diagram showing Earth’s water cycle: the complex and continuous process of water transformation, transport, and storage across various locations on Earth. Source: United States Geological Survey

 

And many of the processes involved – shown in Figure 1 above – are affected by climate change. In fact, the Intergovernmental Panel on Climate Change (IPCC) explains that “the consequences of climate change on terrestrial ecosystems and human societies are primarily experienced through changes to the global water cycle”^[1,2]. For example, they explain that “having too much or too little water increases the likelihood of flooding and drought, as precipitation variability increases in a warming climate”^[1].

Earlier we introduced the concept of water scarcity – but how is this possible on a planet that is roughly 71% covered in water? Most of it – roughly 97% – is stored in the oceans as salt water. Terrestrial freshwater (i.e., water in and on land) represents less than 2% of all water on Earth. And of Earth’s total freshwater, only 4% is considered easily accessible and available for the needs of society^[1].

Although the rate at which freshwater moves through its natural cycles (i.e., through evaporation, precipitation, storage, etc.) is theoretically enough to meet human needs globally (Figure 2), seasonal and geographical variations can alter regional availability^[1]. And these regional changes can impact human lives, as there are already high levels of threat to water security for approximately 80% of Earth’s population^[1,2]. But what drives these changes?

Figure 2 – Diagram showing the flux (movement) of water through Earth’s water cycle in cubic kilometers (km³) per year. Earth. Source: IPCC (2021)^[1]

 

Water scarcity occurs when the supply or availability of freshwater drops below human demands. This can occur during droughts, for example, when conditions are abnormally dry – usually due to low precipitation (e.g., rainfall, snow, hail, etc.). However, periods with excess precipitation can also have negative consequences, such as flooding^[1]. Though these two conditions are seemingly opposite – one wet and one dry – they have something in common: both can be worsened by climate change. While this may at first seem counterintuitive, projections show that as global warming progresses, wet regions will become wetter and dry regions will become drier^[1].

This finding has been used as a target by some climate deniers who claim it is contradictory on the assumption that the effects should be uniform – e.g., everywhere must have more drought OR more precipitation if they are being caused by climate change. However, this assumption is a ‘black or white’ fallacy – an argument using ‘either or’ reasoning – which neglects the real-world complexity of Earth’s processes which are not actually bound to that rule. As we will detail later, scientific evidence shows water cycle impacts can vary by region – a common trait of many climate change effects – with some getting wetter, and others drier. In the sections to follow, we will explain why this occurs and what impacts scientists expect climate change to have on Earth’s water cycle in the future.

 

What scientists know about the relationship between climate change and the water cycle

 

While the effects of climate change on the water cycle are complex, climate scientists have a strong understanding of the physical basis for these effects and robust evidence for their connections.

Their confidence in the relationship between climate change and the water cycle is rooted in unequivocal evidence of what sparks these changes. There is robust evidence that human greenhouse gas emissions have driven global temperature increases in recent times, which we have extensively covered in the past (see links here, here, and here). Although some regional and local impacts need further scientific investigation^[1], there are many effects of climate change on the water cycle that are already known with high confidence. For example, the IPCC explains that “without large-scale reduction in greenhouse gas emissions, global warming is projected to cause substantial changes in the water cycle at both global and regional scales (high confidence)”^[1]. Below we will explain the physical basis for this finding.

In a 2021 report the IPCC outlined the physical basis for the effects of a warming climate on the water cycle. One effect is an increase in severity in both wet and dry events due to increased anthropogenic (i.e., human caused) radiative forcing – increases in global temperatures from greenhouse gas emissions^[1]. This causes a global mean increase in both precipitation (wet events) and evaporation (dry events) through both immediate and longer term feedback effects^[1].

They also explain that “a warmer climate increases moisture transport into weather systems, which, on average, makes wet seasons and events wetter”^[1]. This occurs because the ability of the near-surface atmosphere to hold water increases with temperature at a rate of roughly 7% per 1°C increase^[1]. Following increases in atmospheric moisture from radiative forcing, Earth’s climate response affects precipitation and evaporation, as shown in Figure 3. Note that there is an instant response to increased radiative forcing, followed by atmospheric adjustments and feedback responses. At first, this can result in reduced global precipitation, and later lead to increased precipitation, which highlights the complexity of the relationship between global warming and water cycle changes.

 

Figure 3 – Diagram showing the steps that Earth’s climate undergoes in response to increased radiative forcing (i.e., from human greenhouse gas emissions). These steps alter precipitation patterns over time. Source: IPCC (2021)^[1]

 

Finally, as greater warming occurs over land than over the ocean, atmospheric circulation patterns can change and decrease relative humidity in some regions, increasing drought severity there^[1] (Figure 4).

Figure 4 – Regions (orange) where drought is expected to worsen under future global warming. The pattern is the same regardless of greenhouse gas emission scenarios, but the magnitude of drought worsens with increased emissions. Source: IPCC (2021)^[1]

 

Which of these effects a specific region experiences is influenced by atmospheric circulation patterns and seasonal variations^[1]. This explains the physical basis for these changes, but what have climate scientists observed so far?

 

Human-caused climate change has affected the water cycle since the 20th century

 

Human-caused climate change has driven several changes in the global water cycle since the 20th century^[1]. These changes include increases in atmospheric moisture, precipitation intensity, and global patterns of aridity (i.e., dry regions)^[1]. “Greenhouse gas forcing has driven increased contrasts in precipitation amounts between wet and dry seasons and weather regimes over tropical land areas”, explains the IPCC, “and a detectable precipitation increase in the northern high latitudes”^[1]. To gain further insights on this issue, we contacted Dr. Murray Scown, Associate Senior Lecturer at Lund University, who explained the following:

 

Murray Scown, Associate Senior Lecturer, Lund University:

“In terms of changes in heavy precipitation since the 1950s, scientists have observed increases in most regions of Europe and Asia, central and eastern parts of North America, southern Africa, and northern Australia … We have high confidence in northern Europe and medium confidence in central North America that anthropogenic climate change is contributing to the changes, in other areas the scientific confidence is low because of the complexity of precipitation drivers—it is difficult to get clear agreement on the cause, but it is likely anthropogenic climate change. Nowhere in the world have scientists observed reductions in heavy precipitation since the 1950s.”

 

While widespread heavy precipitation may seem like a positive outcome for drought-stricken regions that need more water, it can actually result in very negative outcomes for these regions. For example, as explained in an article from NASA Earth Observatory, “flash flooding from heavy rains killed dozens and affected 300,000 people in Ethiopia and Somalia in March 2023”, which occurred following three years of drought conditions.

Recent global warming has also significantly impacted high-mountain regions in particular. From 2010-2019, glaciers lost more mass than any decade in the observational record^[1]. Rising global temperatures have driven increased melting of glaciers and caused seasonal changes to streamflow in low-altitude and high-latitude mountain catchments^[1]. The IPCC explains that “Glacier-melt in response to warming can initially lead to increased runoff volumes, especially in peak summer flows, but they will eventually decline as most glaciers continue to shrink.”^[1] The increase in runoff volumes from melting has already declined for some glacier types; for example there is robust evidence that discharge due to the melting of small glaciers has already reached its maximum and is now in decline^[3]. These changes can have local impacts on water resources and agriculture^[3]. For example, “glacier retreat and snow cover changes have contributed to localized declines in agricultural yields in some high mountain regions, including the Hindu Kush Himalaya and the tropical Andes”^[3].

These trends, and others, have been observed at locations around the world. However, the effects of climate change do not act in isolation, but occur in tandem with other drivers, such as human land use. “Since all the components of the water cycle are connected (and linked to the carbon cycle), changes in land use trickle down to many other components of the water cycle and climate system”^[1], explains the IPCC. Figure 5 shows trends/changes in terrestrial water storage (i.e., water storage on land) and their most likely causes between natural variability, direct human impacts (e.g., land use), and climate change impacts.

Figure 5 – Trends in terrestrial water storage (i.e., water storage on land) and their respective drivers from 2002 to 2016. Changes were observed using Gravity Recovery and Climate Experiment (GRACE) satellites. Sources: IPCC (2021)^[1] and Rodell et al. (2018)^[4]

 

Why are these changes important? Changes in precipitation, glacier runoff, and snowmelt can affect runoff and groundwater recharge, which many regions depend on for food, water, and energy security^[2,5-8]. What will these regions and others encounter in the future?

 

Ongoing greenhouse gas emissions and subsequent climate change will further impact the water cycle

 

By pairing climate models with an understanding of the physical basis for water cycle changes we explained earlier, scientists have projected different climate outcomes based on emission scenarios. The results show that substantial changes to the water cycle are projected to occur at both regional and global scales if greenhouse gas emissions are not reduced^[1]. Some of these changes can be reduced or avoided with lower greenhouse gas emissions, while others are expected under all emission scenarios, as summarized below.

 

Anticipated changes under all greenhouse gas emission scenarios^[1]:

• Increased precipitation extremes in nearly all regions
• Intensification of heavy precipitation associated with cyclones
• Continued melting and loss of glacier mass through the 21st century
• Amplified rainfall variability related the El Niño–Southern Oscillation – a climate phenomenon involving El Niño, La Niña, and Neutral phases
• Decreases in snow cover in the low-elevation areas of high-mountain environments

 

The severity of certain outcomes depends on the amount of greenhouse gases humans emit this century. For example, by the end of the century, global annual precipitation will increase by roughly 2.4% under a low emissions scenario (~1.4°C of warming by 2100*) or roughly 8.3% under a very high emissions scenario (~4.4°C of warming by 2100*)^[1]. In northern latitudes, projected increases in precipitation amount and intensity will also increase runoff (e.g., flow of water on ground surfaces)^[1].

However, as explained, despite increases in precipitation amount and severity – which carry their own negative impacts – some regions are expected to experience an increase in dry conditions. For example, in the Mediterranean, south-western South America, and western North America, future aridification will far exceed the magnitude of change seen in the last millennium”^[1]. Some tropical regions, such as the Amazon basin and Central America, will also likely experience increased aridity^[1]. Overall, “the total land area subject to increasing drought frequency and severity will expand”^[5]. Projected changes in dry days per year and daily precipitation are shown in Figure 6 below.

*Note: Temperature increases are relative to the average global surface temperature from the 1850-1900 time period.

 

 

Figure 6 – Maps of projected seasonal changes in dry days per year (i.e., days with less than 1 millimeter of rain) and daily precipitation intensity (i.e., in millimeters per day) for different climate response (emissions) scenarios from lowest to highest from SSP1-2.6 (lowest) to SSP5-8.5 (highest). Regions with high model agreement (greater than 80% agreement) are colored, those with model agreement less than 80% have diagonal lines added. Source: IPCC (2021)^[1]

 

As IPCC explains, these changes will have global implications: “if emissions of greenhouse gases are not curtailed, about a third of global land areas are projected to suffer from at least moderate drought by 2100”^[1]. Curbing emissions – and thus global warming levels – will decrease water scarcity. In fact, the number of people expected to suffer from water scarcity is roughly cut in half by limiting global warming to 1.5°C instead of 2°C^[1,9].

Glacier loss will also worsen with higher emissions. The IPCC explains that under a high emissions scenario “in regions with mostly smaller glaciers and relatively little ice cover (e.g., European Alps, Pyrenees, Caucasus, North Asia, Scandinavia, tropical Andes, Mexico, eastern Africa and Indonesia), glaciers will lose more than 80% of their current mass by 2100^[3]. Regardless of emission scenarios, many glaciers will disappear entirely^[3].

Levels of certainty in projections and attribution of water cycle changes to human-caused climate change continues to grow, as evidenced by new findings that emerged between the IPCC’s fifth and sixth assessment reports on climate change. The IPCC explains that “evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has further strengthened since AR5”^[10]. As scientists continue studying this relationship, better predictions of the effects at regional and local scales are expected to improve^[1]. These advances, along with additional paleoclimate studies, will also help improve attribution of past water cycle effects in the observational record^[1].

 

How to avoid being misinformed about climate change impacts on the water cycle

 

While many of our readers seek out scientific evidence to answer their curiosities, anyone can fall prey to misinformation, especially on complex topics. As we mentioned in the first section and demonstrated in this article – the effects of climate change on the water cycle are complex. With increases in both drought and precipitation intensity, and effects varying by region, there is plenty of room for confusion. Unfortunately, this makes an easy target for climate deniers who sometimes claim that the science is contradictory and that ‘evidence of climate change would show an increase in drought or precipitation – not both’. For example, in the tweet linked here, a Twitter/X user calls climate change the ‘all purpose villian’ that has been attributed to both drought and heavy rains. However, we already provided evidence above that climate change can have effects on both.

 

These types of arguments are not often backed by scientific evidence, but instead by false assumptions. In this case, the assumption or premise is that ‘climate change effects are always binary’; either more of ‘this’ or more of ‘that’. Given that drought and precipitation are opposite conditions – one wet and the other dry – this is an easy thinking trap to fall into. However, this type of black-or-white fallacy ignores that our climate system is dynamic and complex, especially at a global scale. Unfortunately, this type of fallacy is commonly used by those spreading climate change misinformation. To combat this type of misinformation, ask yourself the following:

 

• Is it possible that this argument or claim is oversimplified? Could they be ignoring that science can be complex?
• Do they cite trustworthy sources such as peer-reviewed scientific papers?

 

If the claim seems like an oversimplification of a complex subject, and no trustworthy sources – such as peer-reviewed papers – are cited, that’s the time to put on your investigation hat and seek better information. If we have not covered the subject ourselves, plenty of reliable resources – such as IPCC reports and reputable scientific journals with peer-reviewed research – offer scientific, evidence-based conclusions that account for the nuance of science and complexity of Earth’s climate.

 

Conclusion

The water cycle is a highly complex and interconnected system that is critical to human survival. However, it is impacted by a number of factors including climate change, direct human activities (e.g., land use), and natural variability. Climate has notably affected Earth’s water cycle since the 20th century, and is projected to continue doing so throughout this century as global temperatures rise due to human greenhouse gas emissions. As greenhouse gases affect Earth’s energy budget through radiative forcing, changes occur in our climate system, including both immediate effects and feedback effects, which can affect precipitation and drought in complex ways. While it may seem counterintuitive, these changes can lead to more intense precipitation and drought – Earth’s complex climate system allows for both to increase. In short, this results in wet regions getting wetter, and dry regions getting drier. Additionally, changes in water storage occur through the melting and transport of water away from glaciers and snowpack in mountainous regions, for example. Scientists have used climate models to project different greenhouse gas emission scenarios and their respective future effects on global temperatures and the water cycle. Some changes occur under all emission scenarios – among others, these include increased precipitation in nearly all regions and increased glacier loss through melting. However, decreasing greenhouse gas emissions can make a strong impact; for example, the number of people expected to suffer from water scarcity is roughly cut in half by limiting global warming to 1.5°C instead of 2°C.

 

SCIENTISTS’ FEEDBACK

 

Andreas Prein, Project Scientist, National Center for Atmospheric Research:

 

SF: How has anthropogenic climate change impacted Earth’s water cycle (e.g., precipitation or drought conditions)? Why has this occurred?

AP:

• “A warmer atmosphere can hold more water vapor. This increases the water holding capacity of the atmosphere, which in turn is already resulting in an increase in extreme precipitation and an increase in surface evaporation enhancing floods and droughts at the same time [Trenberth (2011)].”
  1. Reference: Trenberth KE (2011) Changes in precipitation with climate change. Climate research.
• “A result of the former point is that land regions will more quickly transition from drought conditions to very wet conditions and back. This increase in hydrologic volatility (sometimes called whiplash effect) is something we cleanly see in climate models but is also emerging in observational records^[11].”
• “Additionally, increasing temperatures result in a transition from solid to liquid precipitation (e.g., snow falls as rain) and a melting of the cryosphere. Both are resulting in enhanced runoff from the continents to the ocean and a reduction [of] surface albedo^[12].”

SF: What are some of the main effects that climate change is projected to have on the water cycle in this century?

AP: “We expect that all of the effects under point 1 are accelerating with continuous climate change. Many of these effects are directly or indirectly related to temperature increases meaning that every additional degree of warming will further amplify e.g., drought and floods^[13].”

SF: What problems will humans likely face as a result of these projected changes in the water cycle?

AP:

• “Droughts might onset more quickly and frequently^[14].
• Floods have already increased in frequency and intensity^[15].
• Enhanced wildfire risk in many land regions including regions in high latitudes^[16].
• Reduced river base flow due to the melting and disappearance of mountain glaciers^[17].
• Agricultural production might become more challenging due to the increase in extreme weather events [Malhi et al. (2021)]
• More flash flooding in urban regions^[18].
• More challenges in managing water resources (e.g., in reservoirs) due to the higher hydrologic volatility^[19].”

 

Benjamin Cook, Associate Research Scientist, Columbia University:

 

SF: How has anthropogenic climate change impacted Earth’s water cycle (e.g., precipitation or drought conditions)? Why has this occurred? What are some of the main effects that climate change is projected to have on the water cycle in this century?

BC: “Fundamentally, warming caused by anthropogenic greenhouse gas forcing effectively “accelerates” the hydrologic cycle because more energy is available for all the processes that drive different aspects of the hydrologic cycle: precipitation, evaporation, etc. Perhaps somewhat surprisingly, this can mean different changes in different regions and seasons, depending on the underlying baseline climate and dynamics.

 

(1) Annual mean precipitation is expected to increase in the tropics and higher latitudes and decrease in the subtropics (e.g., the Mediterranean). Some of this is through climate change driven changes in circulation and shifts in regions with rising motion and moisture convergence (enhancing precipitation) and areas of sinking motion that diverge moisture (suppressing precipitation). This is a general pattern, but it’s important to note for some regions the uncertainty is large and there can be big differences in the same region across different seasons.

 

(2) Extreme precipitation (e.g., heavy rainfall events) is also likely to increase almost everywhere, because a warmer atmosphere can hold more moisture. So it means more water is available to precipitate when it does start raining during these big storms.

 

(3) At the same time, over most land areas the vapor pressure deficit (VPD; a measure of how dry the atmosphere is) is expected to increase. This is because, while a warmer atmosphere can hold more moisture, in many places on land there is not enough water to keep up with this increasing demand. The air therefore becomes drier (less saturated), which can have all sorts of impacts on ecological process, wildfire, and drought. For example, many areas are likely to experience drier soils because a more arid atmosphere (higher VPD) will effectively suck more moisture out of the soils, making soils drier even if precipitation does not change.

 

(4) In warmer areas with snow, warmer temperatures will also likely mean less precipitation falling as snow and more as rain, earlier melting of snowpacks in the spring, and changes in the seasonality of runoff.

 

To be clear, these are the really broad strokes and, as I mention in (1), you can get big exceptions regionally and seasonally, depending on what other feedbacks and interactions are important. I think the Summary for Policymakers from the most recent IPCC report provides a good overview on a lot of these things (e.g., Figures SPM.5, SPM.6, and SPM.9).”

 

SF: What problems will humans likely face as a result of these projected changes in the water cycle?

 

“Ultimately, in terms of the hydrologic cycle, humans will likely have to simultaneously adapt to situations of both ‘too much’ and ‘too little’ water, though the magnitude impacts will depend on a myriad of social and political factors. For example, while a drought in western North America hurts the livelihoods of farmers and may increase food prices, a drought in a more vulnerable community (e.g., East Africa) can mean famine. With that in mind, areas experiencing more frequent or intense droughts may see reductions in agricultural productivity and ecosystem health, increases in wildfire, and even impacts on the power grid (e.g., less water available for hydropower). In areas where extreme precipitation is increasing, this can mean worse flooding events as drainage systems get overwhelmed.”

 

Murray Scown, Associate Senior Lecturer, Lund University:

 

SF: How has anthropogenic climate change impacted Earth’s water cycle (e.g., precipitation or drought conditions)? Why has this occurred?

MS: “While it is relatively easy to observe and understand how anthropogenic climate change has impacted temperatures on Earth, getting a clear understanding of the impacts on the Earth’s water cycle is more complicated. However, thanks to the latest IPCC Assessment Report 6, we have a very comprehensive synthesis of the current science on this.

Paragraph A.3.2 in Working Group I Summary for Policy Makers (AR6 WG1 SPM) states: “The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis (high confidence), and human-induced climate change is likely the main driver. Human-induced climate change has contributed to increases in agricultural and ecological droughts in some regions due to increased land evapotranspiration (medium confidence).”^[20]

In terms of changes in heavy precipitation since the 1950s, scientists have observed increases in most regions of Europe and Asia, central and eastern parts of North America, southern Africa, and northern Australia. You can view the map here.

We have high confidence in northern Europe and medium confidence in central North America that anthropogenic climate change is contributing to the changes, in other areas the scientific confidence is low because of the complexity of precipitation drivers—it is difficult to get clear agreement on the cause, but it is likely anthropogenic climate change. Nowhere in the world have scientists observed reductions in heavy precipitation since the 1950s. In most of Africa and South America we don’t have enough data and empirical studies to make conclusions.

We’ve seen an increase in agricultural and ecological drought conditions since the 1950s in most of sub-Saharan Africa, Western Europe, the Mediterranean region, Central and Eastern Asia, western North America, eastern South America, and southern Australia. There are two climatic causes of agricultural and ecological drought: reduced precipitation (meteorological drought) and increased evapotranspiration. Scientists have medium confidence that anthropogenic climate change has contributed to the increase in evapotranspiration, particularly in the Mediterranean region and western North America. Only in northern Australia have scientists observed a decrease in agricultural and ecological drought conditions since the 1950s. The effects of anthropogenic climate change on meteorological drought are less well understood because of its complexity.

In terms of tropical cyclones, paragraph A.3.4 in AR6 WG1 SPM states: “It is likely that the global proportion of major (Category 3–5) tropical cyclone occurrence has increased over the last four decades, and it is very likely that the latitude where tropical cyclones in the western North Pacific reach their peak intensity has shifted northward; these changes cannot be explained by internal variability alone (medium confidence). There is low confidence in long-term (multi-decadal to centennial) trends in the frequency of all-category tropical cyclones. Event attribution studies and physical understanding indicate that human-induced climate change increases heavy precipitation associated with tropical cyclones (high confidence), but data limitations inhibit clear detection of past trends on the global scale.”^[20]

There are two major reasons anthropogenic climate change contributes to more intense tropical cyclones. First, tropical cyclones require warm sea surface temperatures to form and it is unequivocal that anthropogenic climate change has warmed the oceans. Second, when there is more water vapor in the air, there is more latent energy available to power the storm. Warmer air holds more water vapor and it is unequivocal that anthropogenic climate change has warmed the atmosphere (air).

Another important component of the water cycle are the monsoons. Untangling the anthropogenic effects on the monsoons is difficult—there are mixed effects of aerosols on cooling and weakening the monsoons, on the one hand, and greenhouse gases warming and intensifying them. Paragraph A.3.3 in AR6 WG1 SPM states: “Decreases in global land monsoon precipitation from the 1950s to the 1980s are partly attributed to human-caused Northern Hemisphere aerosol emissions, but increases since then have resulted from rising GHG concentrations and decadal to multi-decadal internal variability (medium confidence). Over South Asia, East Asia and West Africa, increases in monsoon precipitation due to warming from GHG emissions were counteracted by decreases in monsoon precipitation due to cooling from human-caused aerosol emissions over the 20th century (high confidence). Increases in West African monsoon precipitation since the 1980s are partly due to the growing influence of GHGs and reductions in the cooling effect of human-caused aerosol emissions over Europe and North America (medium confidence).”^[20]

 

SF: What are some of the main effects that climate change is projected to have on the water cycle in this century?

MS: “Overall, IPCC AR6 WG1 SPM states: “Continued global warming is projected to further intensify the global water cycle, including its variability, global monsoon precipitation and the severity of wet and dry events.”^[20] (paragraph B.3).

Scientists have high confidence that every additional 0.5°C of warming will discernibly increase the intensity and frequency of heavy precipitation events, as well as agricultural and ecological drought in some regions. Paragraph B.2.4 in WG1 SPM states: “At the global scale, extreme daily precipitation events are projected to intensify by about 7% for each 1°C of global warming (high confidence). The proportion of intense tropical cyclones (Category 4–5) and peak wind speeds of the most intense tropical cyclones are projected to increase at the global scale with increasing global warming (high confidence).”^[20]

With 1.5°C of warming, a heavy precipitation event that occurred over land, on average, once in 10 years during the period from 1850-1900 would now likely occur 1.5 times every 10 years and be more than 10% wetter. With 4°C of warming, the event would likely occur 2.7 times every 10 years and be more than 30% wetter. (You can see the figure [linked] here)

Regions expected to experience the most severe relative reductions in soil moisture (important for agricultural and ecological drought) include the Mediterranean regions, southern Africa, Central America, the Amazon basin, Patagonia, central and western North America, and parts of China and Mongolia. You can see the map here. ”

 

SF: What problems will humans likely face as a result of these projected changes in the water cycle?

MS: “Depending on the location, problems will include increased frequency and intensity of droughts and floods. Intense storms will continue to cause loss of life, destruction of infrastructure, damage to crops, coastal erosion, and so on. There are many problems with drought, storm, and flood hazards that people have always faced, but the frequency and intensity of events is on the rise. People dependent upon snow-melt and glacier-fed rivers could also have their water supplies severely disrupted.”

 

Ella Gilbert, Research Scientist, British Antarctic Survey:

 

SF: How has anthropogenic climate change impacted Earth’s water cycle (e.g., precipitation or drought conditions)? Why has this occurred?

EG: “Climate change has intensified the planet’s water cycle because as the atmosphere warms, it can retain more water vapour. This effect is described by the Clausius-Clapeyron relationship (Clausius, 1850; Clapeyron, 1834). This means that for every 1 degree celsius that the atmosphere warms, the moisture holding capacity of the atmosphere increases by 7%.

In practice this means more precipitation in some regions, especially those with steep terrain, such as the Antarctic coast or mountain regions, and more intense downpours. It also causes shifts in the monsoon regions because climate change increases the monsoon intensity, and in some places makes rainfall less predictable [see links here and here].”

 

SF: What are some of the main effects that climate change is projected to have on the water cycle in this century?

EG: “Climate change is projected to continue altering the water cycle, leading to further changes in rainfall patterns over tropical and mid-latitudes, and intensifying precipitation. In the polar regions, additional snowfall may initially offset ice losses and sea level rise from certain regions of the ice sheet, but this effect will wane throughout the century as melting begins to outpace snowfall increases.”

 

SF: What problems will humans likely face as a result of these projected changes in the water cycle?

EG: “Changes to the water cycle can result in extreme events like droughts and floods. For example, the World Weather Attribution network has shown that climate change has made several droughts and floods more likely. These events have severe impacts for people’s lives and livelihoods.”

 

REFERENCES

• 1 – IPCC (2021) Water Cycle Changes. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.
• 2 – IPCC (2021) Freshwater Resources. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.
• 3 – IPCC (2019) Special Report on the Ocean and Cryosphere in a Changing Climate.
• 4 – Rodell et al. (2018) Emerging trends in global freshwater availability. Nature.
• 5 – IPCC (2021) 2021: Technical Summary. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
• 6 – Oki and Kanae (2006) Global Hydrological Cycles and World Water Resources. Science.
• 7 – Schewe et al. (2014) Multimodel assessment of water scarcity under climate change. Proceedings of the National Academy of Sciences.
• 8 – Mekonnen and Hoekstra (2016) Four billion people facing severe water scarcity. Science Advances.
• 9 – IPCC (2018) Impacts of 1.5ºC Global Warming on Natural and Human Systems. In: Global Warming of 1.5°C.
• 10 – IPCC (2023) Climate Change 2023: Synthesis Report.
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