Global Warming Is Responsible For Extreme Rainfall

Summary.  This is the second of two posts concerning the relationship between global warming and extremes of rainfall.  The first post,  immediately preceding this one, presents a simplified explanation of how increasing atmospheric temperature leads to increased moisture in the air, and discusses the interplay between water vapor, clouds, heating and cooling of the air, and winds.

This post discusses three recent articles in the scientific journal Nature.  The first two present new results that for the first time directly link extremes of rainfall and flooding to the warming of the average global temperature as a result of greenhouse gas emissions.  Statistical analyses of rainfall and flooding were used to correlate observed rainfall patterns or flooding resulting from heavy rains to the predictions of several climate models.   The third article is a commentary appearing in the same issue of the journal.  According to the commentary, the increased levels of greenhouse gases in the atmosphere, arising from human activity, are likely already adversely affecting the intensity of rainfall and increasing the likelihood of serious damage from flooding.

Introduction.  Humans around the planet continue to burn fossil fuels as their primary energy source at an ever-increasing pace.  Upon combustion, these fuels release carbon dioxide, CO2, a principal greenhouse gas.  Other, more potent greenhouse gases also arise from natural processes and other human activities.  The increased level of CO2 in the atmosphere already added by human activity has led to a global increase in the long-term, planet-wide, average temperature of about 0.7 ̊ C (1.3 ̊ F) above the level that prevailed before the industrial revolution began.  Global warming models predict that higher global temperatures will affect the climate of different parts of the planet in different ways, with some areas experiencing greater amounts of rainfall, and others experiencing drought.

The seemingly increasing severity of extreme weather patterns around the globe in recent years suggests that there may be a correlation with the documented increase in global temperature. Increased rainfall with consequent flooding has been observed in some regions, while increased aridity, sometimes accompanied by increased numbers and extents of wildfires, has also been observed.  Because of the anecdotal nature of such observations, it has been difficult to draw direct correlations between warming of global temperature and localized, somewhat short-term, more extreme excursions of weather.

Analysis of Data Over 49 Years Shows That Global Warming Is Responsible for Extremes of Rainfall Over the Northern Hemisphere.  Min and coworkers (Nature, 2011, Vol. 470, pp. 378-381, doi:10.1038/nature09763; see Note 1) present the first objective identification that human-induced global warming contributes to increased occurrence of extreme precipitation.  They studied rainfall records collected over the 49 year period from 1951-1999 from most regions of the Northern Hemisphere including India and Southeast Asia.  They compared the data with predictions from climate simulations prepared from several distinct models.

Their analysis shows that “human-induced increases in greenhouse gases have contributed to the observed intensification of heavy precipitation events found over approximately two-thirds of … Northern Hemisphere land areas.”  Additionally, the authors observed that the models generally predicted less extreme precipitation than was actually reported in the data, indicating that future projections of the effects of global warming using these climate models may underestimate the occurrence of extreme precipitation.  This underestimate could have serious consequences in planning, both with respect to policies and practices, for future effects of global warming. 

The results are consistent with results of earlier workers that show that the extremes of precipitation are in accord with increased water vapor content that warmer air holds (see the preceding post).  Significantly, the observation period under scrutiny ended in 1999.  This date precedes the even more extreme planetary warming that has been recorded in the last decade, as well as the numerous anecdotal extreme weather events that have been newsworthy in recent years.

Details.  The authors obtained the daily records of rainfall observed at 6,000 stations in the Northern Hemisphere from 1951-1999.  Climate model simulations incorporating the effects of human-induced warming from several computer-driven models were also prepared.  Using a statistical method that identifies extreme deviations of the observed data over the full time interval from the aggregated model predictions, global maps of the Northern Hemisphere were prepared showing the geographic distribution of positive and negative extremes over the entire time interval of the study

Next, occurrences of extreme precipitation summed over the full Northern Hemisphere were prepared at five-year intervals and compared with the various model predictions.  The effects of human-induced warming show increasing trends of occurrence of extreme events over the period, but the predictions were lower in extent than the observations, a trend especially apparent for the last 5-year point.

Finally, using a statistically rigorous optimal detection method, the authors prepared a single number with its uncertainty range, for each of four sub-regions in the Northern Hemisphere.  This number reflects whether characteristics of extreme precipitation occur in the sub-region over the full time period.  The results show that human-induced influences on climate are detected in the observed time-based, and the observed time- and geographic-based, deviations giving rise to extremes of precipitation.  

Extreme Rainfall and Catastrophic Flooding in England and Wales in 2000 Was Very Likely Due to Human-Induced Global Warming.  Pall and coworkers carried out a probabilistic analysis of weather patterns and likelihood of flooding in the region (Nature, 2011, Vol. 470, pp. 382-385, doi:10.1038/nature09762; see Note 1). During October and November 2000 England and Wales had the heaviest rainfall since records began in 1766, leading to severe flooding.  It is estimated that flood damage covered by insurance reached about US$ 2.1 billion.   The authors modeled the rainfall in the area during that period, and then, instead of trying to calculate the flooding directly, they further modeled the groundwater runoff arising from the rain as a substitute for the flooding.  They concluded “it is very likely that global [human-induced] greenhouse gas emissions [occurring during the twentieth century] substantially increased the risk of flood occurrence…in autumn 2000.” 

Details.  The authors sought to obtain the probability of a correlation between rainfall observed, the modeled climate conditions predicted from human-induced global warming, and the extent of flooding.  To begin with, they modeled the rainfall based on the increased humidity predicted from global warming such as described in the preceding post, and compared that with the amount that would have occurred in the absence of the warming of the atmosphere.  It was determined that excess precipitation occurred with a probability of 33%.

The authors then modeled autumn conditions across the Northern Hemisphere for the years 1957-1999, just preceding 2000, compared to four model calculations for the absence of global warming based on conditions presumed to have occurred in 1900.  With the results they developed a three-dimensional representation over the hemisphere up through the atmosphere.  (This type of time comparison has been explained in an early Warmgloblog post.)   This result was compared to a similar calculation for the year 2000.  The pattern differed considerably from that of the preceding decades.

Next, as a substitute for measuring the flooding itself, the authors modeled the runoff of rain falling in the watersheds (catchbasins) of the rivers in the area, using geological and climate simulations, on a day-by-day basis.  The amount of computation that this required was so huge that they accomplished it using distributed computing on volunteered computers throughout the Internet.  For the single model including global warming, and for each of the four models for the absence of global warming, over 2000 simulations were computed.  For each of the models for no warming, scatter plots of simulated results portraying daily runoff were displayed, and compared to the single complete model including warming.  The points computed for the complete model showed runoff values higher than all computed points for three of the no warming models, and for a large majority of computed points for the fourth no warming model.  These differences became especially pronounced for computed simulations that correspond to times for recurrence of rare events (i.e., the expected interval between recurrences) of greater than 10 autumns.

Smoothed histograms of a “fraction of risk attributable to twentieth century [human-induced] greenhouse gases” display the probabilities for the occurrence of the excess runoff.  The four separate histograms were then combined into a single composite.  The composite shows that the increase in risk that the floods under consideration were due to increased human-induced greenhouse gas emissions is very likely, i.e., found in 9 out 10 cases, with a risk of more than 20%, and to be likely, i.e., found in 2 out of 3 cases, with a risk of more than 90%.

A Commentary on the Two Articles Described Above Is Provided by Allan, who was not involved in either study, in the same issue of Nature; (see Note 1) .  Allan points out the important role played by the increase in water vapor capacity of air with increasing temperature.  He also notes that water vapor itself is thought by some to affect atmospheric behavior, acting to lessen precipitation due to radiative cooling of the atmosphere.  As a consequence the ultimate increase in potential global rainfall may be less than 6% per  ̊ C implied from the water vapor capacity alone.  Allan also points to the need for more study on possible local effects in storms and otherwise in more limited regions that can act to increase or to diminish the temperature dependence of rainfall resulting from changes in the water vapor capacity of air.

Conclusion.  The two research articles discussed here are the first to show rigorously, using computational statistical methods and planetary climate models, that increased occurrence of heavy rainfall arises as a direct consequence of the increase in atmospheric concentration of greenhouse gases due to human activity.  This increased rainfall includes severe extremes that inflict major damage to human society.  It is important to extend these types of statistical analysis to other warming-related phenomena.

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Note 1. Abstract available online free, or the full article for a fee or through personal or institutional subscription.  Many public libraries, and university libraries open to the public, receive the journal.

© 2011 Henry Auer

Ice, Water and Water Vapor

Summary.  This is the first of two posts concerning the relationship between global warming and extremes of rainfall.  The second post, immediately following this one, presents summaries of two research articles and a commentary on them, that for the first time attribute extreme rainfall and an instance of severe flooding directly to human-induced global warming due to greenhouse gas emissions.

Here a simplified discussion of the transformations between ice, water, and water vapor is presented, as a tutorial.  First, these changes are considered in closed containers which are surrounded by air at various temperatures.  The interchange of heat energy needed to bring about the changes in state of the water is emphasized.  Next, in order more closely to resemble water vapor in the atmosphere, the water-to-vapor transformation is discussed without a container separating the water from the air, but rather mixed together.  Again, the energy interchange is stressed.  Finally, we discuss very simply how these transformations might be involved in creating rainfall, and winds or larger patterns of weather.

Introduction. In the physical world, a substance can exist either as a solid (at low temperature), a liquid (at a higher temperature) or a gas or vapor (at a still higher temperature).  An example familiar to us all is water.

Solid-Liquid-Vapor Phase Changes. Here, to start with (please see Note 1) let’s talk only about pure water, so we’re not including mixtures or solutions of water or its vapor with other substances.  Why is solid water, ice, found at low temperatures, liquid water at intermediate temperatures and the gaseous form, water vapor, at the highest temperatures?  There are two aspects to an answer. 

Heat Energy. First, think about the ice itself, or the liquid water in a closed container or the water vapor in a closed container, surrounded by air for example. There is less heat contained in the air when its temperature is low, and more heat contained in it when its temperature is high.  Heat is one form of energy.  The air can transfer some of its heat energy to the ice or to the water or vapor through the containers, or in reverse the ice or water or vapor can transfer some of its heat energy to the air surrounding it.  (Heat can only flow from a warmer to a colder temperature.)

Intrinsic Energy. Second, the physical states of solid, liquid and gas themselves contain differing amounts of intrinsic, or latent, energy.  The ice contains the least, the water more, and the water vapor the most (see the graphic below). 

Liquid Water-Vapor Interconversion. The same processes occur when liquid water is vaporized to a gas.  The molecules in the liquid water still have some bonds between the molecules, but not as many as there were in the ice.  The molecules in the water vapor have essentially no bonds between them, i.e., they are isolated.  In order to convert from liquid water to the pure water vapor inside the container, more heat energy has to be provided from the surrounding air to break the remaining bonds between the molecules in the liquid, and permit them to escape from the liquid as independent molecules of water vapor (right, upper arrow).  (For simplicity, the graphic ignores the fact that the vapor and its container are much larger than the liquid.)  Thus the water vapor inside the container has still more intrinsic, or latent, heat than the liquid water. This process is also reversible.   If the air were to cool down enough, the water molecules in the vapor would release their intrinsic heat to the surrounding air, condensing back into the liquid (right, lower arrow).

Transfer of heat energy. It’s crucial to understand from this explanation that the more important process is the transfer of heat energy between the surroundings and the substance.  As people, we can see the change from ice to liquid water, and from liquid water to vapor, and back.  But we don’t see the energy driving these visible changes.  It’s actually the transfer of the heat energy that’s important, even if we can’t see it. 

When we put an ice cube into a drink, the liquid of the drink transfers its heat to the ice, supplying the intrinsic heat needed to melt the ice.  But as a result, the liquid drink has less heat energy remaining, i.e., it has cooled down.  We observe the melting ice cube, and the cooling of the drink.  But we don’t directly sense the heat transfer.

A Model: Compressed Gas and Gravity.  It might be helpful to think of the following idealized setup as a model.  Suppose a marble is at the bottom of a sloping ramp between two horizontal platforms (see the graphic below, left).  A cylinder and piston contain a gas under high pressure, and the piston rod extends out of the cylinder, with a cup that can push the marble up the ramp from the low platform to the higher platform.  Here, the low platform is a model for our solid, ice, and the higher platform is a model for our liquid, water.  If the gas in the cylinder is allowed to expand, the piston pushes the marble from the lower platform (ice) up the ramp to the higher platform (liquid water; see the graphic, right).  The expanding gas transfers its energy of compression to the marble in the form of intrinsic gravitational energy.  The marble on the upper platform has more gravitational (latent) energy than it had on the lower platform (upper arrow).  

This process is reversible (lower arrow).  Let’s suppose that gravity, or some additional force, could make the marble roll down the ramp, giving up its intrinsic gravitational energy.  The lost gravitational energy goes to increasing the compression energy of the gas by pushing the piston back into the cylinder again.  In this example, the energy of compression of the gas is exchanged back and forth with the gravitational energy of the marble.

Liquid Water and Water Vapor in the Atmosphere. Now let’s return to focus on water, and only on the exchange between liquid water and gaseous vapor.  Also, we’ll now open up the container so that air can be mixed with any water vapor that escapes from the liquid.  This is a good way to think of liquid water and water vapor in the earth’s atmosphere.  On a molecule-by-molecule basis, to a first approximation the intrinsic energy to vaporize a molecule of water from the liquid is more or less the same at all temperatures.  

Since the liquid water and the water vapor are no longer enclosed in a container, some liquid water can transfer to water vapor and mix with the air, at all temperatures.  But between the melting point of ice and the boiling point of water, there is a 100 °C (180 °F) difference in temperature.  So as the temperature of the water and the air above it gets warmer, more heat can be supplied to the liquid water.  More molecules of water acquire the intrinsic heat needed to vaporize, and more water vapor enters the air over the surface of the liquid water.  In other words, as the environment’s temperature increases, the overall capacity of the air above liquid water to hold water vapor increases. 

Roughly, at temperatures at which the earth’s weather is determined, a change of 1 °C (1.8 °F) in temperature changes the capacity of the air to hold water vapor by about 6% (Note 2).  This means that if the temperature increases by 1 °C, an amount of heat energy must be supplied to provide the intrinsic energy for 6% more water to vaporize into the air if that air is saturated with water vapor; if the temperature falls by 1 °C, the water content of the air at saturation falls by about 6%, releasing that intrinsic heat energy back into the air.  This intrinsic, or latent, heat of vaporizing liquid water, or condensing water vapor, is actually quite high.  It is much higher than for other chemical compounds related to water, such as ammonia (Note 3) or hydrogen fluoride.

Global Warming and its Effect on Water Vapor Content of the Atmosphere.  Global warming scenarios predict some areas on the surface of the earth having more rainfall, and more violent storms, than prior to the onset of warming.  At present, the long-term increase in global temperature is 0.7 °C (1.3 °F) higher than the temperature prior to the start of the industrial revolution.  Water can evaporate from the surface of the earth from oceans, fresh water lakes and rivers, and from moist lands and green fields and forests.  Oceans occupy about 71% of the surface of the earth.  Evaporation from the surface of the oceans and land masses contributes a significant amount of water vapor in the atmosphere.  At the current level of the increase in global temperature, a simplified global average of the increase in the amount of moisture in the atmosphere could be about 4% more than before warming began (Note 4). 

Our discussion above should make plausible the way that global warming can lead to weather patterns with more rainfall in some regions.  A higher moisture content in the air provides the potential, upon cooling, for more moisture to condense into precipitation, by releasing its intrinsic heat content to the cooler air.  In this way global warming is understood to lead potentially to heavier rainfall in some areas.  (These comments do not represent all the factors that are included in modeling global warming.  They simply strive to convey some of the simpler processes that are involved in translating global warming into altered patterns of climate and weather.)

Cloud Formation and Loss, and Heat Transfer.  Now consider what you see when you look at a cloud, observing its ever-changing shape and appearance.  The cloud that is visible is actually made of condensed droplets of liquid water, i.e., water that has already released its intrinsic heat to the surrounding air, warming the air up in the process.  In contrast, water vapor in the air surrounding the cloud is itself invisible.  A cloud sometimes loses a part of its wispy condensate, vaporizing back into water vapor, thereby absorbing its intrinsic heat for vaporization from the surrounding air, and cooling the air.  This simplified description should help us understand that cloud formation and loss involve important localized changes in air temperature and heat content. 

Global Warming and Winds.  In the preceding paragraph, we discussed how water vapor and cloud formation involve reciprocal exchanges of heat energy, with cycles of warming and cooling of the air.  On a larger scale, when air masses of differing temperatures encounter each other, one result is generation of wind (wind also has other origins).  In certain conditions we could imagine that more turbulent winds could arise from global warming because of the increased moisture content and the resulting increase in exchanges of intrinsic heat with the heat of the air.  This could produce a tendency for stronger winds, some of which could lead to more violent storms.  (Other factors not considered here can also contribute to this tendency.)

Conclusion.  Here we’ve shown that transformation of water between ice, liquid and vapor is accompanied by significant exchanges of heat with the air in the immediate environment of the changes.  In addition, the capacity of the atmosphere to hold water vapor increases with the higher global average temperature that prevails as a result of global warming.  This enhances the intensity of the heat exchange processes, which potentially can lead to more, and stronger winds, and more, and more intense precipitation as snow or rain.  It is possible that global warming exacerbates the damaging effects of severe precipitation.  The following post discusses this in greater detail.

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Note 1.  This presentation is not meant to be comprehensive or technically accurate.  It is intended to make plausible the ways in which global warming can affect certain changes in weather phenomena.

Note 2. Handbook of Chemistry of Physics, 53^rd Ed., 1972-1973, CRC Press, Cleveland, OH, page D-148.

Note 3. Handbook of Chemistry of Physics, 53^rd Ed., 1972-1973, CRC Press, Cleveland, OH,  page E-21.

Note 4. This simplified statement ignores differences that might arise between the tropics and the poles, and the possible effects from the fact that some polar oceans are covered with ice.

© 2011 Henry Auer