Simulating the Biogeochemical and Biogeophysical Impacts of Transient Land Cover Change and Wood Harvest in the Community Climate System Model (CCSM4) from 1850 to 2100

Abstract

To assess the climate impacts of historical and projected land cover change in the Community Climate System Model, version 4 (CCSM4), new time series of transient Community Land Model, version 4 (CLM4) plant functional type (PFT) and wood harvest parameters have been developed. The new parameters capture the dynamics of the Coupled Model Intercomparison Project phase 5 (CMIP5) land cover change and wood harvest trajectories for the historical period from 1850 to 2005 and for the four representative concentration pathway (RCP) scenarios from 2006 to 2100. Analysis of the biogeochemical impacts of land cover change in CCSM4 reveals that the model produced a historical cumulative land use flux of 127.7 PgC from 1850 to 2005, which is in general agreement with other global estimates of 156 PgC for the same period. The biogeophysical impacts of the transient land cover change parameters were cooling of the near-surface atmosphere over land by 20.18C, through increased surface albedo and reduced shortwave radiation absorption. When combined with other transient climate forcings, the higher albedo from land cover change was counteracted by decreasing snow albedo from black carbon deposition and high-latitude warming. The future CCSM4 RCP simulations showed that the CLM4 transient PFT parameters can be used to represent a wide range of land cover change scenarios. In the reforestation scenario of RCP 4.5, CCSM4 simulated a drawdown of 67.3 PgC from the atmosphere into the terrestrial ecosystem and product pools. By contrast the RCP 8.5 scenario with deforestation and high wood harvest resulted in the release of 30.3 PgC currently stored in the ecosystem.

Interactions between land use change and carbon cycle feedbacks

“…Our estimates of harvest carbon loss are thus about twice as high as estimates from the forcing model [Hurtt et al, 2011] as described in Lawrence et al [2012b], and the impact of this on our conclusions is discussed more in section 4.…”

“…The harvesting rates used here are higher than estimated in the forcing data, which might make up for the low land use conversion rates (section 2.1; [Hurtt et al, 2011;Lawrence et al, 2012b]), although this difference could also be due to differences in methodology [e.g., Hansis et al, 2015]. In any case, the results here suggest that carbon feedbacks in coupled models are quite sensitive to the LULCC time series prescribed in a particular scenario.…”

“…Note that although Lawrence et al [2012b] modified the harvesting rates input to the CESM to better reproduce the original estimates from Hurtt et al [2011], these modifications were not incorporated into the simulations conducted for CMIP5 and thus were not used here or in other analyses using the same model configuration [Lindsay et al, 2014;Randerson et al, 2015]. Our estimates of harvest carbon loss are thus about twice as high as estimates from the forcing model [Hurtt et al, 2011] as described in Lawrence et al [2012b], and the impact of this on our conclusions is discussed more in section 4.…”

“…Transient land cover is prescribed over the historical and future time period through 2100 using the RCP8.5 scenario [Hurtt et al, 2011], converted into plant functional types (pfts) appropriate for implementation into this model [Lawrence et al, 2012a[Lawrence et al, , 2012b. After 2100, the land use conversion is assumed to stop (i.e., the distribution of plant functional types remains constant), while harvesting is maintained at a constant rate.…”

“…We consider the direct effect of land use conversion and harvesting on carbon, as well as a potential indirect impact, which is an interaction of changes in LULCC with climate, rising CO 2 , and nitrogen deposition (see Figure 1; we use some of the nomenclature from Gitz and Ciais [2003] and Pongatz et al [2009], but the details of our calculation are different). Note that there are many different approaches used in the literature for reporting land carbon stock changes, and thus we try to be clear here which processes we are including [e.g., Houghton, 2003;Gasser and Ciais, 2013;Lawrence et al, 2012b;Hansis et al, 2015]. The direct fluxes are by definition the change in carbon stocks that occur during 1 year from the conversion of natural lands to managed lands.…”