Total equivalent area of Agricultural Land and Urban Surfaces (>55%) in the Continental U.S.
In the battle over global warming, it is sometimes said that humans are too small to make a significant impact on global climate. However, surveys of land use and land cover changes over the last decade suggest that the amount of land altered by human activity is large enough to modify climate on a regional, continental, and even hemispheric scale.
In the U.S., as in most countries, the largest alterations have been made by agriculture. Nearly 54% of the continental U.S. is agricultural land. Though much smaller (about 1.5%), urban surfaces exert an influence proportionally greater than their size. Globally, 38% of the world’s land area is used for agriculture and about 0.5% consists of urban surfaces.
This is a conservative estimate which does not include surface modifications such as private, commercial, and municipal landscaping, deforestation, watersheds, surface mining, landfills, reservoirs, etc. When all such modifications are taken into consideration, global landscape changes approach 40%-50% of the total landmass.
These vast modifications to the land can significantly alter temperature, evaporation, cloud cover, precipitation, pressure fields, and wind over a region—and perhaps beyond. Unfortunately, their impact has been largely ignored by many in the climate community owing to a disproportionate emphasis on the role of CO2 in climate change.
Urban Heat Islands (UHI)
In a little known study published by NOAA in 2004, scientists used Landsat data, satellite observed nighttime lights, U.S. Census Bureau road vectors, and high resolution aerial photographs to create a map of Impervious Surface Area (ISA) for the continental U.S. ISA consists of human constructed surfaces including buildings, roads, parking lots, roofs, airports, etc.
In 2004 the total ISA was 112,610 km2 (± 12,725 km2) which is slightly smaller than the state of Ohio (116,534 km2) as shown on the map below.
In a 2007 study, another survey was undertaken by some of the same researchers to tabulate the global constructed surface area. Due to the size of the survey area, a lower resolution model was used which, as the lead author indicated to me in an email, probably underestimates ISA (e.g., for the continental U.S. the estimate of 83,337 km2 is 26% lower than the 2004 study).
The total ISA for the top 100 countries is 579,703 km2. China, the United States and India have the largest constructed surface area by far, totaling 252,284 km2. Assuming this estimate is 26% lower than the actual value, the world wide total would be closer to 783,330 km2—an area slightly larger than Turkey (780,580 km2).
Impervious surfaces have a major impact on local and mesoscale climate by altering sensible and latent heat fluxes. According to recent studies, the rapid growth of urban areas may account for 50% of the warming in the U.S. since 1950 (Stone, 2009, see Fall 2009).
ISA also modifies watersheds by increasing the frequency and magnitude of surface runoff pulses and raising water temperatures. Nearly all runoff is removed from impervious surfaces by storm sewers, thus reducing available moisture. But higher water temperature increases the potential for evaporation of standing water.
A large UHI can actually establish its own circulation pattern when conditions are favorable. As dry warm air rises over the city, it is replaced by a cool, moist inflow from the surrounding countryside. This sets up a low level convergent flow which is favorable for cumulus formation and thunderstorm development downwind.
Convergence is enhanced by surface roughness. As air slows due to increased friction, the continued inflow causes mass to accumulate. Equilibrium is maintained as updrafts form to remove mass. Frictional drag also tends to divert air around the city which then converges again on the downwind side producing more lift.
Studies done in the St. Louis area during the 1970’s showed a 25% average increase in summer rainfall. This included a 58% increase in nocturnal rainfall. Deep convective activity from the increased heat fluxes resulted in a 45% increase in thunderstorm frequency and a 31% increase in hailstorms. Thunderstorms have been observed to persist and reform for hours downwind of large urban areas.
Since thunderstorms are a major mechanism for transporting heat and moisture deep into the troposphere, the cumulative impact of UHIs on regional and continental climate has yet to be determined. The upshot is that UHIs not only cause an increase in average temperature, but they may also have teleconnections to synoptic scale phenomena.
Agricultural Land
Unlike UHIs, agricultural lands are coextensive, covering thousands of square kilometers. The surface area covered by agricultural land is large enough to create regional and continental scale modifications to climate.
The 2007 “U.S. Census of Agriculture” issued by the USDA, reported that in 2002 there were 2.2 million farms, covering an area of 4,148,027 km2.
Consequently, 54% of the contiguous U.S.(which is approximately 7,689,027 km2) has been modified by croplands, grassland pastures, and ranges as shown in the census table below. 223,850 km2 of U.S. agricultural land was irrigated in 2003—an area slightly smaller than the state of Minnesota.
| Table 1.1.1 Major uses of land, United States, 20021 | ||||
| Land use | 48 States | United States | 48 States | United States |
| Million acres | Percent of total | |||
| Cropland2 | 441 | 442 | 23.3 | 19.5 |
| Grassland pasture and range | 584 | 587 | 30.8 | 25.9 |
| Forest-use land3 | 559 | 651 | 29.5 | 28.8 |
| Special uses4 | 153 | 297 | 8.1 | 13.1 |
| Urban | 59 | 60 | 3.1 | 2.6 |
| Miscellaneous other land | 97 | 228 | 5.1 | 10.1 |
| Total land area5 | 1,894 | 2,264 | 100.0 | 100.0 |
| 1See Major Land Uses for estimates of major uses by region and State, coinciding with each census of agriculture, from 1945 through 2002. See the glossary for detailed definitions of the major land uses. | ||||
| 2All land in the crop rotation (used for crops, used for pasture, idle cropland). Includes about 34 million acres idled under the Conservation Reserve Program. | ||||
| 3Total forest land as classified by the U.S. Forest Service minus an estimated 98 million acres of forested land used for parks, wildlife areas, and other special uses. | ||||
| 4Rural transportation areas, land used primarily for recreation and wildlife purposes, various public installations and facilities, farmsteads, and farm roads/lanes. Excludes urban land in contrast to Major Land Uses, Aggregate Data. | ||||
| 5Distributions by major use may not add to totals due to rounding. | ||||
| Sources: USDA/ERS based primarily on reports and records of the Census Bureau and Federal, State, and local land management and conservation agencies. See the Major Land Uses report for information about the 2002 land-use estimates. | ||||
Globally 38% of the world’s land area is used for agriculture (crops, pastures, and range). This amounts to 56,597,200 km2—an area slightly larger than Asia, the Middle East, and Europe combined (World Bank). Worldwide, there is 2,788,000 km² of irrigation equipped infrastructure (year 2000 figures)—an area slightly larger than Argentina.
Total equivalent area of Global Agricultural Land
Numerous studies have shown that there is often a reduction in near surface temperature (both day and night) when land is converted to agriculture due to the increase in plant transpiration. In other words, a greater percentage of solar energy is now used to change liquid water into vapor making less energy available for heating the ground.
The same is true of evaporation from irrigated land. Irrigation causes a sharp gradient in temperature between irrigated and non-irrigated areas. Using satellite derived data, one study measured IR temperature gradients of 10ºC -12ºC over distances of 10-20 km in Northeast Colorado (see Cotton & Pielke, 118-121).
As with UHIs, a large temperature gradient is sufficient to set-up a local solenoidal circulation. Landscape heterogeneities on the order of tens of kilometers are sufficient to produce a mesoscale circulation. Convergent zones associated with these circulations can become the focus of deep thunderstorm convection, though in some cases (e.g., the Indian monsoons) reduced surface roughness over croplands can actually inhibit deep convection.
Increased evaporation and transpiration over agricultural land adds a considerable amount of moisture to the air raising dew point temperatures. This moisture can enhance cloud formation and even thunderstorm development as latent heat is released from condensing water vapor. The magnitude of latent heat flux is enormous in terms of heat content or moist enthalpy. A 1ºC increase in dew point temperature is equivalent to a 2.5ºC increase in air temperature (at a pressure of 1000mb). The result is an increase in convective available potential energy (CAPE) for cumulus and thunderstorm growth (though initiation may be delayed by evaporative cooling in the surface boundary layer) and an increase in precipitation.
These impacts undergo seasonal changes especially in dryland agriculture such as wheat. The leafing out of vegetation in the spring causes cooling while the harvest in the fall may actually produce a warming effect. Crop rotation, conversion to pastureland, desertification, deforestation, and overgrazing of ranges also produce warm anomalies.
The immense spatial scale of agricultural land leads to the palpable conclusion that anthropogenic modifications to land cover are altering temperature, evaporation, cloud cover, precipitation, pressure fields, and wind over entire regions and continents. While these modifications usually act to modulate natural climate variability, on some spatial and temporal scales they may actually dominate certain parameters. Agricultural activity has produced large scale changes in average rainfall (Florida, the Midwest, North America, and India), decreases in average summer temperatures, and increases in average summer dewpoints (irrigated land, Midwest cornbelt).
The challenge for the future is to further demonstrate how changes in land use and land cover teleconnect to synoptic scale phenomena and to untangle the complex biophysical, bigeochemical, and biogeographic interactions created by changes in soil and vegetation. This includes the impact made by aerosols as well as CO2 sinks and sources.
References:
2007 US Census of Agriculture. http://www.agcensus.usda.gov/Publications/2007/Full_Report/index.asp
Lubowski, Ruben, Marlow Vesterby, and Shawn Bucholtz (2006-07-21). Agricultural Resources and Environmental Indicators, 2006 Edition. USDA Economic Research Service. “AREI Chapter 1.1: Land Use”
World Bank Database. http://data.worldbank.org/topic/agriculture-and-rural-development
“The Digital Global Map of Irrigation Areas – Development and Validation of Map Version 4” Siebert, Stefan, Jippe Hoogeveen, Petra Döll, Jean-Marc Faurès, Sebastian Feick and Karen
Frenken Tropentag. University of Bonn, October 11-13, 2006, Conference on International Agricultural Research for Development. http://www.tropentag.de/2006/abstracts/full/211.pdf
CIA World Factbook, “irrigation.” https://www.cia.gov/library/publications/the-world-factbook/fields/2146.html
Cotton, William R., and Pielke, Sr., Roger A. Human Impacts on Weather and Climate, 2nd Edition (Cambridge U. Press, 2007).
Elvidge, C. D., C. Milesi, J. B. Dietz, B. T. Tuttle, P. C. Sutton, R. Nemani, and J. E. Vogelmann (2004), U.S. Constructed Area Approaches the Size of Ohio, Eos Trans. AGU, 85(24), doi:10.1029/2004EO240001 http://www.ngdc.noaa.gov/dmsp/pubs/ElvidgeEtAl-EOS-ConstructedAreaApproachesSizeOfOhio.pdf
Summary in AGU News: http://www.agu.org/news/press/pr_archives/2004/prrl0423.html
Elvidge, C.D.; Tuttle, B.T.; Sutton, P.C.; Baugh, K.E.; Howard, A.T.; Milesi, C.; Bhaduri, B.; Nemani, R. Global Distribution and Density of Constructed Impervious Surfaces. Sensors 2007, 7, 1962-1979. http://www.mdpi.com/1424-8220/7/9/1962/pdf
Cotton, William R., and Pielke, Sr., Roger A. Human Impacts on Weather and Climate, 2nd Edition (Cambridge U. Press, 2007).
Pielke Sr., R.A., 2005: Land use and climate change. Science, 310, 1625-1626.
Stone, Brian, Land Use as Climate Change Mitigation. Environ. Sci. Technol., 2009, 43 (24), pp 9052–9056 http://pubs.acs.org/doi/abs/10.1021/es902150g
Fall S., D. Niyogi, R. A. Pielke Sr., A. Gluhovsky, E. Kalnay and G. Rochon, 2009: Impacts of land use land cover on temperature trends over the continental United States: assessment using the North American Regional Reanalysis, International Journal of Climatology. http://www.landsurface.org/publications/J80.pdf
Climate Science: Roger Pielke Sr. 
