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要旨:
The impacts of land use have been shown to have considerable influence
on regional climate. With the recent international commitment to limit
global warming to well below 2 degrees C, emission reductions need to be
ambitious and could involve major land-use change (LUC). Land-based
mitigation efforts to curb emissions growth include increasing
terrestrial carbon sequestration through reforestation, or the adoption
of bioenergy crops. These activities influence local climate through
biogeophysical feedbacks, however, it is uncertain how important they
are for a 1.5 degrees climate target. This was the motivation for
HAPPI-Land: the half a degree additional warming, prognosis, and
projected impactsland-use scenario experiment. Using four Earth system
models, we present the first multimodel results from HAPPI-Land and
demonstrate the critical role of land use for understanding the
characteristics of regional climate extremes in low-emission scenarios.
In particular, our results show that changes in temperature extremes due
to LUC are comparable in magnitude to changes arising from half a degree
of global warming. We also demonstrate that LUC contributes to more than
20% of the change in temperature extremes for large land areas
concentrated over the Northern Hemisphere. However, we also identify
sources of uncertainty that influence the multimodel consensus of our
results including how LUC is implemented and the corresponding
biogeophysical feedbacks that perturb climate. Therefore, our results
highlight the urgent need to resolve the challenges in implementing LUC
across models to quantify the impacts and consider how LUC contributes
to regional changes in extremes associated with sustainable development
pathways.
Plain Language Summary The motivation for the Intergovernmental Panel on
Climate Change Special Report of 1.5 degrees C stems from the need to
understand how the impacts of climate change may evolve for half a
degree of global warming. Most low-emission scenarios involve
substantial land-use change (LUC) including the expansion of bioenergy
and food crops, as well as afforestation. Future emission scenarios used
as input to climate models are derived using integrated assessment
models, and focus on greenhouse gas emissions. However, changes in land
use also have a direct effect on local climate through the local water
and energy balances, which is not considered in these models, and
therefore, our understanding on how dependent these climate projections
are to the choice of land-use scenario is limited. Our study
demonstrates that the land-use scenario has a considerable influence on
the projections of temperature extremes for low-emission scenarios. In
particular, for large land areas in the Northern Hemisphere, more than
20% of the change in temperature extremes can be attributed to LUC.
However, our study also reveals that considerable uncertainty remains on
what the feedbacks of land use may mean for land-based mitigation
activities.
