Land degradation is a systemic global problem, but the
scale of the problem is disputed, with global estimates of
degraded areas ranging from <10 to >60 million km2
. Changes in
vegetation in drylands are predominantly caused by two factors:
(i) anthropogenic climate change, which includes both changes in
water availability driven by trends in precipitation and increases
in temperature, as well as increased water use efficiency (carbon
gain per unit of water lost) in response to rising atmospheric CO2;
and (ii) land use practices, including grazing, cropping and
deforestation. Unsustainable land use is considered the primary
negative driver of dryland degradation. The impact of climate
change on drylands is also generally thought to be negative, with
some studies suggesting that anthropogenic forcing has already
increased arid areas.
Despite evidence for land use-induced degradation and the
studies that find increased aridification over drylands, satellite
estimates of vegetation greenness show a significant global
increase since 1980. The key drivers of this global increase in
apparent vegetation productivity are the vegetation’s response to
rising CO2, increases in rainfall and temperature and land use.
Model simulations which prescribe land use, attribute almost all
of the trend in satellite-derived greening to CO2 fertilization,
while satellite-derived models that do not account for CO2,
explicitly find either climate or land use as the dominate factor.
Neither approach explicitly accounts for rapid ecosystem change
in their proportioning of the relative contributions of each driver.
This can lead them to miss or underestimate rapid changes driven
by processes like extreme fires, deforestation, reforestation,
changes in agricultural policy, etc. Disentangling the roles of
climate (temperature and precipitation), CO2 and land use thus
remains a key challenge.
A.L. Burrell; J.P. Evans; M.G. De Kauwe. Anthropogenic climate change has driven over 5 million km2
of drylands towards desertification. Internet:<www.sciencedirect.com> (adapted)