Land’s complex role in climate change

To fully appreciate the scope and diversity of climate change, one first needs a clear understanding of what is meant by climate. As illustrated in figure 1, the climate system consists of the atmosphere, oceans, land, and the ice- and snow-covered regions known collectively as the cryosphere. Each system component can be characterized by certain state variables— atmospheric temperature, ocean salinity, land moisture, and depth of snow cover, to name a few.

Climate change occurs when a perturbation—a forcing, as it’s known in Earth-science parlance—generates a flux that alters the components’ natural states. A 2005 US National Research Council report more precisely defines climate change as any multidecadal or longer alteration in one or more physical, chemical, or biological state variables or fluxes within the climate system.⁴ The time scale of several decades is typically used to distinguish climate change from short-term variations in weather and other aspects of climate.

Of course, Earth’s climate is always changing, so in a sense, the “change” in climate change is redundant. In addition to anthropogenic contributions, climate is subject to both internal forcings—from ocean currents, atmospheric currents, and the like—and external forcings such as solar variability and volcanic activity. The term “change,” however, is generally used by policymakers to imply change resulting from human actions.

Global warming, the increase in the average heat content of the climate system, is one type of change. It is best characterized in terms of the spatially integrated temperature of the ocean—where more than 90% of heat change occurs—and is expressed in units of joules. A two-dimensional global average of surface temperature trends is therefore an imperfect metric to diagnose global warming. It’s also inadequate to characterize the many facets of climate change.

Arguably, the aspects of climate that most affect us and our environment at local and regional scales are those that influence weather patterns—droughts, floods, tropical cyclones, heat waves, and so forth. (Sea-level rise is one regionally important aspect of climate that is not directly linked with weather.) Weather patterns are influenced primarily by regional atmospheric and ocean circulations such as El Niño, the North Atlantic Oscillation, and the Pacific Decadal Oscillation. Changes in those circulations should be of even greater concern than globally averaged properties. (See the article by Thomas Birner, Sean Davis, and Dian Seidel, Physics Today, December 2014, page 38.)

Regional weather patterns are, in part, a function of land cover. Modifications to the biophysical characteristics of the land and to the way we manage it can alter the relative abundances of carbon, nitrogen, and other trace gases and aerosols near Earth’s surface and the fluxes of water, heat, light, and momentum between components of the climate system.^5,6 To see how land-cover change can directly impact climate, let’s consider an example: irrigation.

The irrigation of semiarid land can dramatically alter a region’s water balance.⁷ Due to the combined effects of evaporation and transpiration, collectively termed evapotranspiration, increases in ground moisture tend to raise humidity in the overlying atmosphere. Such increases in humidity can mean the difference between a mild shower and a torrential downpour.

To understand how, consider the concept of moist static energy, $S$⁠, which can be expressed as

The term $CpT$ represents sensible heat—the heat derived from an increase in temperature $T$⁠. The term $Lq$ represents latent heat, a potential energy that’s stored in the vapor phase during the process of evaporation. Here, $Cp$ is the specific heat of water at a constant pressure, $L$ is water’s latent heat of vaporization, and $q$ is specific humidity—the mass ratio of water vapor to air.

In essence, $S$ is a measure of the buoyant potential energy. Thunderstorms feed on that energy; the larger it is, the more intense a storm can become. The irrigation of a dry patch of land boosts $S$ by increasing $q$⁠.

The effect can be dramatic: At atmospheric pressure, a mere 1 °C rise in dew point from 23 °C to 24 °C—equivalent to an increase in $q$ of 1 g/kg—would have the same effect on $S$ as a 2.5 °C rise in temperature. (The same humidity increase would have a smaller effect in a cooler atmosphere and a larger effect in a warmer one.) The increase in moist static energy due to humidity would be partially offset by a reduction in $T$ from the cooling effects of evapotranspiration, and the chances of triggering a thunderstorm by surface heating would also decrease. However, if a thunderstorm were to form, it would likely be more intense and produce more precipitation.

Figure 2 shows an illustrative case study: a weather simulation of 7000 km² of farmland, brush, and shortgrass in the Great Plains region of the US.⁸ In a simulation using that terrain as it was on 15 May 1991, evapotranspiration generates enough atmospheric moisture and moist static energy to produce heavy rainfall and thunderstorms. But in a scenario in which the terrain is covered by only dry shortgrass—as it was before human influences—no storms form.

Examples of large-scale, human-driven landscape transformations are increasingly easy to find. The human footprint on the land is vast and expanding, driven largely by the growth and intensification of agriculture.⁹ (See figure 3.) Global cropland cover is estimated to have increased from 300 million hectares to 1530 million hectares between the years 1700 and 2000.

Even in some areas where the land cover has not changed, the manner in which the land is used has. Between 1700 and 2000, for example, the global area used for grazing livestock increased 10-fold, from 324 million hectares to 3429 million hectares. By 2000 only a few desert regions, the central Amazon and Congo basins, arid areas of Australia, and the Arctic and Antarctic had not been significantly affected by humans. Roughly half of Earth’s land surface is estimated to have suffered intensification in land use.¹⁰ Such shifts in land management can drastically alter carbon, heat, and water fluxes between the surface and the overlying atmosphere. (See the box on page 45.)

The growing human footprint on the landscape is disquieting in part because of the sheer magnitude of its effect. In parts of Arizona, urbanization-induced warming—commonly known as the urban heat-island effect—could boost temperatures by up to 7 °C in coming decades, nearly three times the predicted rise in temperature attributable to greenhouse gas emissions.¹¹ (See figure 4 for a visual illustration of the heat-island effect in London.)

Changes in land cover and management have large impacts at local and regional scales even when their average global effect is small. Unlike added CO₂, which has a globally homogenous effect on radiation, changes in land cover and land management can produce large spatial variations in climate-system fluxes. Those variations are the driving forces disturbing local and regional weather patterns.

The weather effects are all the more concerning because they are often spatially coherent over large scales. The alteration of land at one location may influence weather patterns at distant locales through atmospheric couplings known as teleconnections. The hypothesized mechanism is analogous to that by which El Niño, the periodic warming of the eastern and central Pacific Ocean, affects weather thousands of kilometers away. Although the theory is still being explored, teleconnections are thought to link land use to changes in the polar jet stream, the paths of tropical cyclones, and the frequencies and intensities of droughts, floods, heat waves, and other weather events.

The Land Use Model Intercomparison Project, led by David Lawrence of the National Center for Atmospheric Research and George Hurtt of the University of Maryland, is one of several current efforts to improve our understanding of teleconnections and land’s role in the climate system. The researchers are urging modeling groups to examine those and other issues of land-related climate forcings.

In 2009, 19 fellows of the American Geophysical Union concluded that

A few years later, scientists at the University of New South Wales made the case that the human influence on extreme temperatures cannot be assessed by CO₂ levels alone because at the regional scale, land cover and land management can enhance or mask effects from greenhouse gases.¹³ Also, global averaging tends to obscure land-change effects, which can depend on geographical region, latitude, and the previous state of the landscape. Yet anthropogenic greenhouse gas emissions have remained the primary focus of multidecadal climate models such as those that informed the debate that resulted in the 2015 Paris agreement.

However, are CO₂ levels and global averaged surface temperature sufficient to generate accurate and meaningful forecasts? Two leading hypotheses have emerged.

The first argues that the accuracy of climate forecasts emerges only at time periods beyond a decade, when greenhouse gas emissions dominate over other human forcings, natural variability, and influences of initial value conditions. The hypothesis assumes that changes in climate are dominated by atmospheric emissions of greenhouse gases, of which CO₂ is the most important. It represents the current stance of the Intergovernmental Panel on Climate Change and was adopted as the basis of the Paris agreement.

A second hypothesis is that multidecadal forecasts incorporating detailed initial value conditions and regional variation set an upper bound on the accuracy of climate projections based primarily on greenhouse gas emissions. According to that view, successful models must account for all important human forcings—including land surface change and management—and accurately treat natural climate variations on multidecadal time scales. Those requirements significantly complicate the task of prediction.¹⁴ 

Testing the hypotheses must be accomplished by using “hindcast” simulations that attempt to reproduce past climate behavior over multidecadal time scales. The simulations should be assessed by their ability to predict not just globally averaged metrics but changes in atmospheric and ocean circulation patterns and other regional phenomena.

The climate system and our role in it are complex. Not only is climate influenced by human forcings, but those forcings are influenced by a host of societal and environmental factors, including population growth, personal consumption levels, and property-value trends. (See the article by Paul Higgins, Physics Today, October 2014, page 32.) How then should we assess vulnerabilities and mitigate risks of future climate change?

In our view, the approach should be a comprehensive one that accounts for the full range of forcings, not just carbon emissions. It should incorporate risks from human modification of the land, including the effect of land use on weather patterns.¹⁴ And it should adopt a resource-based vulnerability framework: Identify climate, environmental, and societal threats to critical water, food, health, energy, and ecosystem resources; then optimize mitigation and adaptation strategies according to the relative risks of the various threats to each resource. (See figure 5.)

In the American Geophysical Union’s 2012 monograph on extreme events and natural hazards, several scientists—including one of us (Pielke)—recommended a resource-based framework focused on local and regional scales as a more inclusive approach for policymaking.¹⁵ Compared with conventional approaches, which start from the global-climate-model perspective, resource-based frameworks are better equipped to deal with the diversity and complexity of societal and environmental threats faced at the community level.

A resource-based vulnerability framework would give policymakers a better perspective not only of how human forcings affect climate but of how climate affects risk. For example, restricting development of land in flood plains and in coastal locations affected by hurricane storm surge is an effective adaptation strategy regardless of how climate changes.

At present, land is being considered too narrowly in the development of climate policy. To help reduce unintended impacts of land use on climate, we recommend the following:¹⁶ 

► Translating international treaties and protocols into national policies and actions that deliver positive climate outcomes and reduce the spectrum of risks to key societal and environmental resources.

► Updating international protocols to reflect new scientific understanding of the role of land in the climate system.

► Continuing to invest in the measurement, database development, reporting, and verification of land-use and land-management activities while monitoring effects of those activities on the climate system and linking them to emissions-reduction efforts.

► Adding developed countries to the Reducing Emissions from Deforestation and Forest Degradation protocol, which currently covers only developing countries.

As Earth’s population has boomed, human forcings have become an increasingly important driver of climate. Current climate-change mitigation policies do not sufficiently incorporate the effects of changes in land surface and land management on the surface albedo, the fluxes of heat and moisture in the atmosphere, and the distribution of energy within the climate system.¹⁷ Given the goal of mitigating climate change at local, regional, and global scales, it won’t suffice to frame the problem simply in terms of greenhouse-gas-induced warming; one must consider threats posed by the entire climate system—and work toward a fuller understanding of that system.

To be sure, incorporating land-cover change and land management complicates attempts to address climate change through, say, a system of credits and debits such as those being considered for fossil-fuel emissions and carbon sequestration. However, recognition of the complexity of anthropogenic climate change does not absolve us of the responsibilities to understand and minimize our impact on Earth’s climate system and to reduce societal and ecological vulnerability to environmental change of all types. The problem is formidable, but so are the tools, technologies, and resources with which we can tackle it.

We thank Udaysankar Nair of the University of Alabama in Huntsville for providing the box figure and Dallas Staley of the Cooperative Institute for Research in Environmental Sciences in Boulder, Colorado, for her invaluable feedback and edits to the manuscript.

Roger Pielke is a senior research scientist at the Cooperative Institute for Research in Environmental Sciences in Boulder, Colorado, and a professor emeritus at Colorado State University in Fort Collins. Rezaul Mahmood is a professor in the department of geography and geology at Western Kentucky University and associate director of the Kentucky Climate Center, both in Bowling Green. Clive McAlpine is a professor in the school of geography, planning, and environmental management at the University of Queensland in Brisbane, Australia.