An introduction to climate change - Climate Change: Biological and Human Aspects - page 32

that 20th-century climate change alone is insufficient to account for run-off changes.
However, such an analysis of a surface run-off model has indicated that including the
suppression of carbon dioxide-induced stomatal closure makes the model's outcome
consistent with actual run-off data (Gedney et al., 2006; Matthews, 2006). In short,
biology plays a key part in controlling the water cycle to such a degree that without
it climate change and other non-biological factors cannot fully explain water-cycle
trends.
Yet, as we shall see throughout this topic, there is much that is unexpected in the
detail of climate and its interactions with the biosphere. For example, as mentioned,
in a warmer world we would expect more ocean evaporation, hence more rainfall
and more plant evapotranspiration. This is not always so, even if the underlying
trend is one of a warmer world leading to more ocean evaporation. In 2010 a large
international team involving many research departments, and led by Martin Jung,
provided a data-driven estimate of global land evapotranspiration from 1982 to 2008,
compiled using a global monitoring network, and meteorological and remote-sensing
observations. 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 as expected. On average the
increase was by 7.1 1.0 mm per year per decade from 1982 to 1997. All well and

good, but after that - coincident with the last major El Ni no event in 1998 - the global
evapotranspiration increase seems to have ceased until 2008. This change was driven
primarily by moisture limitation in the southern hemisphere, particularly Africa and
Australia. In these regions, microwave satellite observations indicate that soil mois-
ture decreased from 1998 to 2008. Hence, increasing soil-moisture limitations on
evapotranspiration largely explain the recent decline of the global land evapotranspir-
ation trend. Not all the extra ocean evaporation in a warmer world results in increased
rain uniformly distributed over the land. So, whether the changing behaviour of
evapotranspiration is representative of natural climate variability or reflects a more
permanent reorganisation of the land water cycle is a key question for Earth system
science.
As we shall see again and again throughout this topic, climate and biology are
connected: they affect each other. Furthermore, humans, as a biological species that
also relies on and which is in turn affected by other biological species, are caught in
the middle of this climate-biology dynamic. That human action is also a significant
driver of current climate change further complicates matters. We need to understand
both how our species affects the global climate and the climate-biology dynamic
if our growing population is to survive without a decline in either well-being or
environmental quality.

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1.8 Fromtheorytoreality

In the above review of the causes of climate change it can be seen that climate
theories, whether of greenhouse climatic forcing or Milankovitch's orbital effects
of incoming solar energy, have to be validated. This is done by comparing theory