finding 10.1 : key-finding-10-1

Changes in land use and land cover due to human activities produce physical changes in land surface albedo, latent and sensible heat, and atmospheric aerosol and greenhouse gas concentrations. The combined effects of these changes have recently been estimated to account for 40% ± 16% of the human-caused global radiative forcing from 1850 to present day (high confidence). In recent decades, land use and land cover changes have turned the terrestrial biosphere (soil and plants) into a net “sink” for carbon (drawing down carbon from the atmosphere), and this sink has steadily increased since 1980 (high confidence). Because of the uncertainty in the trajectory of land cover, the possibility of the land becoming a net carbon source cannot be excluded (very high confidence).

This finding is from chapter 10 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The key finding is based on basic physics and biophysical models that have been well established for decades with regards to the contribution of land albedo to radiative forcing (NRC 2005). Recent assessments specifically address additional biogeochemical contributions of land-cover and land-use change to radiative forcing.6c7c285c-8606-41fe-bf93-100d80f1d17a 64b0d456-bec3-44a8-92d0-92215a3e2e29 The role of current sink strength of the land is also uncertain.71c75d19-f2ad-4bf1-9cb8-b9a08f8c3ef0 e25be0f6-adbc-4a5b-a614-db2dea9ee409 The future distribution of land cover and contributions to total radiative forcing are uncertain and depend on policy, energy demand and food consumption, dietary demands.d6ca431b-0c90-40e6-bb8d-47e5e293198a

Description of evidence base: Traditional methods that estimate albedo changes for calculating radiative forcing due to land-use change were identified by NRC.64b0d456-bec3-44a8-92d0-92215a3e2e29 That report recommended that indirect contributions of land-cover change to climate-relevant variables, such as soil moisture, greenhouse gas (e.g., CO2 and water vapor) sources and sinks, snow cover, aerosols, and aerosol and ozone precursor emissions also be considered. Several studies have documented physical land surface processes such as albedo, surface roughness, sensible and latent heat exchange, and land-use and land-cover change that interact with regional atmospheric processes (e.g., Marotz et al. 1975;0a5f08d8-c318-4276-a578-e52be2ca52ff Barnston and Schickendanz 1984;bad518a7-59b0-4194-864e-18bd2e76c02e Alpert and Mandel 1986;00ba60eb-88b2-4ca5-860d-1af87a8becb2 Pielke and Zeng 1989;52fb1a23-37a8-4e93-b3f9-d697157f2fe7 Feddema et al. 2005;960d836f-c6af-4589-b60c-7e1dfa16a4f8 Pielke et al. 20074bda18ab-f956-498d-a12d-4c6eb7def1a2); however, traditional calculations of radiative forcing by land-cover change in global climate model simulations yield small forcing values (Ch. 2: Physical Drivers of Climate Change) because they account only for changes in surface albedo (e.g., Myhre and Myhre 2003;3ad55175-2951-4a06-abc3-9724acd1ae1e Betts et al. 2007;35f369a8-cad8-4664-872d-2ab2d4ab9553 Jones et al. 201534a398d1-1a9c-489c-b72e-aa91b9c864ed).

Recent studies that account for the physical as well as biogeochemical changes in land cover and land use radiative forcing estimated that these drivers contribute 40% of present radiative forcing due to land-use/land-cover change (0.9 W/m2).6c7c285c-8606-41fe-bf93-100d80f1d17a 0ea16ff3-9026-4a77-9077-742e29f88f9b These studies utilized AR5 and follow-on model simulations to estimate changes in land-cover and land-use climate forcing and feedbacks for the greenhouse gases—carbon dioxide, methane, and nitrous oxide—that contribute to total anthropogenic radiative forcing from land-use and land-cover change.6c7c285c-8606-41fe-bf93-100d80f1d17a 0ea16ff3-9026-4a77-9077-742e29f88f9b This research is grounded in long-term observations that have been documented for over 40 years and recently implemented into global Earth system models.6c7c285c-8606-41fe-bf93-100d80f1d17a 5660257c-9227-40b8-98d5-3fc758ef394a For example, IPCC 2013: Summary for Policymakers states: “From 1750 to 2011, CO2 emissions from fossil fuel combustion and cement production have released 375 [345 to 405] GtC to the atmosphere, while deforestation and other land-use changes are estimated to have released 180 [100 to 260] GtC. This results in cumulative anthropogenic emissions of 555 [470 to 640] GtC.”2ad39d48-c8d4-46cf-9a5c-0bc65a4da57c IPCC 2013, Working Group 1, Chapter 14 states for North America: “In summary, it is very likely that by mid-century the anthropogenic warming signal will be large compared to natural variability such as that stemming from the NAO, ENSO, PNA, PDO, and the NAMS in all North America regions throughout the year”.541fc57b-d7ad-4617-97de-2df91f99afc0

New information and remaining uncertainties: Uncertainty exists in the future land-cover and land-use change as well as uncertainties in regional calculations of land-cover change and associated radiative forcing. The role of the land as a current sink has very high confidence; however, future strength of the land sink is uncertain.71c75d19-f2ad-4bf1-9cb8-b9a08f8c3ef0 e25be0f6-adbc-4a5b-a614-db2dea9ee409 The existing impact of land systems on climate forcing has high confidence.6c7c285c-8606-41fe-bf93-100d80f1d17a Based on current RCP scenarios for future radiative forcing targets ranging from 2.6 to 8.5 W/m2, the future forcing has lower confidence because it is difficult to estimate changes in land cover and land use into the future.d6ca431b-0c90-40e6-bb8d-47e5e293198a Compared to 2000, the CO2-eq. emissions consistent with RCP8.5 more than double by 2050 and increase by three by 2100.b05850d3-a9d7-4395-8588-5e27531c160a About one quarter of this increase is due to increasing use of fertilizers and intensification of agricultural production, giving rise to the primary source of N2O emissions. In addition, increases in livestock population, rice production, and enteric fermentation processes increase CH4 emissions.b05850d3-a9d7-4395-8588-5e27531c160a Therefore, if existing trends in land-use and land-cover change continue, the contribution of land cover to forcing will increase with high confidence. Overall, future scenarios from the RCPs suggest that land-cover change based on policy, bioenergy, and food demands could lead to significantly different distribution of land cover types (forest, agriculture, urban) by 2100.60f8630b-59b0-4ee0-ae04-d5a129253c69 b05850d3-a9d7-4395-8588-5e27531c160a 4b0b096e-c877-46bb-a4d3-95a3dd964fd6 bfd356e4-1e09-47b2-b618-218f59c7d5d3 f9b8e11b-7694-40b3-ae41-bf8cfb290fd8 d6ca431b-0c90-40e6-bb8d-47e5e293198a

This finding was derived from figure -.2: Confidence / Likelihood

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