Are the impacts of land use on warming underestimated in climate policy?
2017-09-18,
Mahowald, Natalie M.,
Ward, Daniel S.,
Doney, Scott C.,
Hess, Peter G.,
Randerson, James T.
While carbon dioxide emissions from energy use must be the primary target of climate change
mitigation efforts, land use and land cover change (LULCC) also represent an important source of
climate forcing. In this study we compute time series of global surface temperature change separately
for LULCC and non-LULCC sources (primarily fossil fuel burning), and show that because of the
extra warming associated with the co-emission of methane and nitrous oxide with LULCC carbon
dioxide emissions, and a co-emission of cooling aerosols with non-LULCC emissions of carbon
dioxide, the linear relationship between cumulative carbon dioxide emissions and temperature has a
two-fold higher slope for LULCC than for non-LULCC activities. Moreover, projections used in the
Intergovernmental Panel on Climate Change (IPCC) for the rate of tropical land conversion in the
future are relatively low compared to contemporary observations, suggesting that the future
projections of land conversion used in the IPCC may underestimate potential impacts of LULCC. By
including a ‘business as usual’ future LULCC scenario for tropical deforestation, we find that even if
all non-LULCC emissions are switched off in 2015, it is likely that 1.5 ◦C of warming relative to the
preindustrial era will occur by 2100. Thus, policies to reduce LULCC emissions must remain a high
priority if we are to achieve the low to medium temperature change targets proposed as a part of the
Paris Agreement. Future studies using integrated assessment models and other climate simulations
should include more realistic deforestation rates and the integration of policy that would reduce
LULCC emissions.
Interactions between land use change and carbon cycle feedbacks
2017-01-23,
Mahowald, Natalie M.,
Randerson, James T.,
Lindsay, Keith,
Munoz, Ernesto,
Doney, Scott C.,
Lawrence, Peter,
Schlunegger, Sarah,
Ward, Daniel S.,
Lawrence, David,
Hoffman, Forrest M.
Using the Community Earth System Model, we explore the role of human land use and land cover change (LULCC) in modifying the terrestrial carbon budget in simulations forced by Representative Concentration Pathway 8.5, extended to year 2300. Overall, conversion of land (e.g., from forest to croplands via deforestation) results in a model-estimated, cumulative carbon loss of 490 Pg C between 1850 and 2300, larger than the 230 Pg C loss of carbon caused by climate change over this same interval. The LULCC carbon loss is a combination of a direct loss at the time of conversion and an indirect loss from the reduction of potential terrestrial carbon sinks. Approximately 40% of the carbon loss associated with LULCC in the simulations arises from direct human modification of the land surface; the remaining 60% is an indirect consequence of the loss of potential natural carbon sinks. Because of the multicentury carbon cycle legacy of current land use decisions, a globally averaged amplification factor of 2.6 must be applied to 2015 land use carbon losses to adjust for indirect effects. This estimate is 30% higher when considering the carbon cycle evolution after 2100. Most of the terrestrial uptake of anthropogenic carbon in the model occurs from the influence of rising atmospheric CO2 on photosynthesis in trees, and thus, model-projected carbon feedbacks are especially sensitive to deforestation.