Advances in understanding large-scale responses of the water cycle to
climate change
Abstract
Globally, thermodynamics explains an increase in atmospheric water vapor
with warming of around 7%/°C near to the surface. In contrast, global
precipitation and evaporation are constrained by the Earth’s energy
balance to increase at ∼2–3%/°C. However, this rate of increase is
suppressed by rapid atmospheric adjustments in response to greenhouse
gases and absorbing aerosols that directly alter the atmospheric energy
budget. Rapid adjustments to forcings, cooling effects from scattering
aerosol, and observational uncertainty can explain why observed global
precipitation responses are currently difficult to detect but are
expected to emerge and accelerate as warming increases and aerosol
forcing diminishes. Precipitation increases with warming are expected to
be smaller over land than ocean due to limitations on moisture
convergence, exacerbated by feedbacks and affected by rapid adjustments.
However, these temperature-dependent changes offset rapid atmospheric
adjustments to radiative forcings which tend to increase precipitation
over land relative to the oceans. These factors therefore drive complex
changes in the regional water cycle in time and space, some examples of
which will be discussed. Thermodynamic increases in atmospheric moisture
fluxes amplify wet and dry events, driving an intensification of
precipitation extremes. The rate of intensification can deviate from a
simple thermodynamic response due to in‐storm and larger‐scale feedback
processes, while changes in large‐scale dynamics and catchment
characteristics further modulate the frequency of flooding in response
to precipitation increases. Changes in atmospheric circulation in
response to radiative forcing and evolving surface temperature patterns
are capable of dominating water cycle changes in some regions. Moreover,
the direct impact of human activities on the water cycle through water
abstraction, irrigation, and land use change is already a significant
component of regional water cycle change and is expected to further
increase in importance as water demand grows with global population.
This talk will summarize recent advances in understanding past and
future large-scale responses in the water cycle.