finding 5.1 : key-message-5-1

Changes in land cover continue to impact local- to global-scale weather and climate by altering the flow of energy, water, and greenhouse gases between the land and the atmosphere (high confidence). Reforestation can foster localized cooling (medium confidence), while in urban areas, continued warming is expected to exacerbate urban heat island effects (high confidence).

This finding is from chapter 5 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

Chapter authors developed the chapter through technical discussions, literature review, and expert deliberation via email and phone discussions. The authors considered feedback from the general public, the National Academies of Sciences, Engineering, and Medicine, and federal agencies. For additional information about the overall process for developing the report, see Appendix 1: Process.

The topic of land-use and land-cover change (LULCC) overlaps with numerous other national sectoral chapters (for example, Ch. 6: Forests; Ch. 10: Ag & Rural; Ch. 11: Urban) and is a fundamental characteristic of all regional chapters in this National Climate Assessment. This national sectoral chapter thus focuses on the dynamic interactions between land change and the climate system. The primary focus is to review our current understanding of land change and climate interactions by examining how land change drives changes in local- to global-scale weather and climate and how, in turn, the climate drives changes in land cover and land use through both biophysical and socioeconomic responses. Where possible, the literature cited in this chapter is specific to changes in the United States.

Description of evidence base:

The Land-Use and Climate, IDentification of robust impacts (LUCID) project9724e954-e516-40f1-9d0c-12396f63bd12,e80f1d68-00df-48cb-8372-c52b36879ba4 evaluated climate response to LULCC using seven coupled land surface models (LSMs) and global climate models (GCMs) to determine effects that were larger than model variability and consistent across all seven models. Results showed significant discrepancies in the effect of LULCC (principally, the conversion of forest to cropland and grassland at temperate and higher latitudes) on near-surface air temperatures; the discrepancies were mainly attributable to the modeling of turbulent flux (sensible heat [the energy required to change temperature] and latent heat [the energy needed to change the phase of a substance, such as from a liquid to a gas]). Land surface models need to be subjected to more rigorous evaluationse80f1d68-00df-48cb-8372-c52b36879ba4,c81bdacc-63c5-461b-a586-b6a33c69fe26 and evaluate more than turbulent fluxes and net ecosystem exchange.c81bdacc-63c5-461b-a586-b6a33c69fe26 Rigorous evaluations should extend to the parameterization of albedo,bb8fc4fd-559b-4ee3-9f73-82aafa1e3805 including the effect of canopy density on the albedo of snow-covered land;46ff7f8c-532b-415c-90a7-724aa440979a the seasonal cycle of albedo related to the extent, timing, and persistence of snow;34366f3e-a66f-4ff4-a4ca-b7101ffacf97 and the benchmarking of the effect of present-day land cover change on albedo.a7c74eca-75d9-4750-aa97-8ec3a87856d6 More recently, there is consistent modeling and empirical evidence that forests tend to be cooler than nearby croplands and grasslands.09c64d74-bd7c-4bc5-bb28-ab597a7b225d,a631c5fc-9d86-4f16-9e84-021d306568cf,0fa5c0aa-9e41-4ce5-8770-f43fd181c61e,edd9b30c-5c5d-4d44-b2b0-38296699ad42,1370a540-a389-4b2f-93aa-e169f3b27ec2,a7c74eca-75d9-4750-aa97-8ec3a87856d6

The study of the influence of wildland fire on climate is at its advent and lacks a significant knowledge base.b541b2e9-ffac-447b-9098-6516f9a7bd8d,1639ecca-a0ca-48f4-b64c-a447fe137284 Improved understanding would require more research on the detection of fire characteristics;9074a177-82cd-490c-a669-dac34268dadb fire emissions;e97dbfb4-6ce6-4329-8389-bcf5a7a06d8a and the relative roles of greenhouse gas (GHG) emissions, aerosol emissions, and surface albedo changes in climate forcing.b541b2e9-ffac-447b-9098-6516f9a7bd8d

The urban heat island (UHI) is perhaps the most unambiguous documentation of anthropogenic modification of climate.71fd8e9a-5d84-4d79-bff0-35da4ee4e999 Two studies have found that the stunning rate of urbanization in China has led to regional warming,994fde97-75e2-462d-8d41-f70b096c01c0,cc8c0a04-4945-4069-a94d-d9b2e3da7aec which is consistent with the observation that land-use and land-cover changes must be extensive for their effects to be realized.a81627c2-d26d-474a-a2ca-e42d6ab0bfac Research on the effects of urbanization on precipitation patterns has not produced consistent results.3a32c45b-bf35-41c1-b86f-a568f54485fd,97afeefb-daa0-4b8b-ad44-7a4414547827 Uncertainties related to the effect of urban areas on precipitation arise from the interactions among the UHI, increased surface roughness (for example, tall buildings), and increased aerosol concentrations.e02f219f-4d94-4aac-8592-c3ca14693aba In general, UHIs produce updrafts that lead to enhanced precipitation either in or downwind of urban areas, whereas urban surface roughness and urban aerosol concentrations can either further contribute to or dampen the updrafts that arise from the UHI.e02f219f-4d94-4aac-8592-c3ca14693aba

New information and remaining uncertainties:

Land use and land cover are dynamic; therefore, climate is influenced by a constantly changing land surface. Considerable uncertainties are associated with land-cover and land-use monitoring and projection.06e194ef-b57f-4a5e-b633-7e58386dcfd8,b8957392-913e-4d32-bfdb-544efbd5ccb9,ff9ffc0a-4f57-4c2f-9137-ecaa8b0fd5ee,3770dd69-0cf4-44c9-84a5-ed62a2e66841 Land-cover maps can be derived from remote sensing approaches, but comprehensive approaches are typically characterized by coarse temporal resolution.437471ba-9fe3-4547-b193-7bf3ec00fbf3,f859d4bf-3716-4300-9779-6f6af8ce4c66,e0e4c3fb-f4f5-42a4-a64a-7e21d8eaa355,00bfab03-4a87-486b-90d2-4707410fe9f9 More recently, remote sensing has enabled annual classification over large areas (national and global), though these efforts have been centered on a single land cover or disturbance type.a763e6c4-f1c0-41b0-954a-524bfdad6300,6a3ce882-e3f6-47c6-a9ae-dacb25c45e7f,c7b0af41-0488-4efa-9216-592dc6f30805 Comprehensive multitemporal mapping of land use is even more limited and is a source of considerable uncertainty in understanding land change and feedbacks with the climate system. Deforestation, urbanization, wildland fire, and irrigated agriculture are the main land-use and land-cover changes that influence climate locally and regionally throughout the United States. Deforestation is likely to behave as a warming agent throughout most of the United States, but higher confidence in this finding would require more research on how to treat sensible and latent heat fluxes in coupled GCM–LSM models; the relationship of albedo to forest density in the presence of snow; the timing, persistence, and extent of snow cover; and real-world comparisons of the response of albedo to land-cover change. Urbanization constitutes a continued expansion of the UHI effect, increasing warming at local scales. Determining the effect of urbanization on precipitation patterns and storm tracks would require extensive, additional research. Tabular irrigation water volume estimates, such as those provided by the U.S. Department of Agriculture’s (USDA) Farm and Ranch Irrigation Survey, must be translated into maps so that the data can be input in GCMs and LSMs to determine the impact of irrigation on climate. Current translation schemes do not provide consistent model output.277a4220-6a9d-4820-8a11-c10dc83b0494 The effect of wildland fires on climate processes is an emerging issue for which there is little research. Fire releases carbon dioxide (CO2) and other GHGs to the atmosphere, which, along with a decreased albedo, should promote warming. These warming effects, however, may be counterbalanced by the release of aerosols to the atmosphere and enhanced carbon sequestration by forest regrowth.1639ecca-a0ca-48f4-b64c-a447fe137284

Assessment of confidence based on evidence:

There is medium confidence that deforestation throughout much of the continental United States promotes climate warming through a decrease in carbon sequestration and reduced transpiration. There is low confidence that wildland fires will impact climate, because many of the associated processes and characteristics produce counteracting effects. There is high confidence that urbanization produces local-scale climate change, but there is low confidence in its influence on precipitation patterns. There is high confidence that surface air temperature is reduced near areas of irrigated agriculture and medium confidence that downwind precipitation is increased.

References :

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