Present climate and biological change - Climate Change: Biological and Human Aspects - page 201

hemisphere's subtropics and tropics, and moistening in the southern hemisphere's
subtropics and deep tropics. Finally, the observed meteorological changes were larger
than those suggested by the models, which says something about the nature of the
change together with the capability of models to simulate the future. This also builds
on preliminary work in 2007 by Frank Wentz and colleagues (see section 1.7).
The year 2007 saw other work corroborating the notion that by the beginning
of the 20th century we were influencing the global hydrological cycle. The autumn
of 2007 saw corroboration of the Wentz team's conclusion that precipitation and
total atmospheric water have increased with warming at about the same rate over
the past two decades, of around 6% ◦ C − 1 . A British group led by Katherine Willett
also looked at meteorological data and compared it with a climate model. They
identified a significant global increase in surface humidity that is mainly attributable
to human influence as distinct from natural forcing. Finally, and importantly as this
text concerns climate change biology, another British team, this time led by Betts, in
2007 used an ensemble of 244 runs of a climate model that incorporated a terrestrial
vegetation dimension. They investigated whether terrestrial vegetation would affect
the hydrological cycle of a warmer world: would a warmer world see more continental
run-off ? (By the early 21st century climate modellers were increasingly using models
with biological components in addition to the physical atmosphere, land and ocean
components that had by the end of the 20th century become commonplace.) There
are two trends in plant physiology that are likely in a world warmed by more carbon
dioxide. First, being warmer (and with enough moisture) plants will transpire more
water. Second, with an increase in atmospheric carbon dioxide, stomata (the openings
on the surface of leaves through which water and gas can pass) will tend to close and/or
become fewer in number: either way, transpiration will be reduced. The question is,
which trend - more or less transpiration - will dominate and how will this affect
continental run-off ? The conclusions of Betts and colleagues were that in some areas
plants will fail to get enough water because of greater transpiration, and that there
will be more evaporation from land, leading to some regions being prone to drought.
Overall, this reduction in transpiration together with the increase in the hydrological
cycle (more water being cycled) will result in increased continental run-off. This
means that there will be more water in streams and rivers, and hence some areas may
see greater flooding.
Together, all the above work illustrates that the global climate system is changing
profoundly and this has implications for the hydrological cycle in the 21st century
and beyond.
As ever, I must sound a word of caution. The end of the 21st century, and a world
that is 4 ◦ C warmer, are a long way off. A lot could happen to global evapotranspiration
between now and then. In 2010 a Western European and US team led by Martin Jung,
Markus Reichstein and Philippe Ciais provided a data-driven estimate of global land
evapotranspiration from 1982 to 2008, compiled using a global monitoring network,
meteorological and remote-sensing observations and the use of a computer model. In
addition, they assessed evapotranspiration variations over the same time period using
an ensemble of process-based land-surface models. Their results suggest that global
annual evapotranspiration increased on average by 7.1 1.0 mm per year per decade

from 1982 to 1997. After that, coincident with the last major El Ni no event in 1998,

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