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| Titel |
A general framework for understanding the response of the water cycle to global warming over land and ocean |
| VerfasserIn |
M. L. Roderick, F. Sun, W. H. Lim, G. D. Farquhar |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 18, no. 5 ; Nr. 18, no. 5 (2014-05-06), S.1575-1589 |
| Datensatznummer |
250120344
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| Publikation (Nr.) |
 copernicus.org/hess-18-1575-2014.pdf |
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| Zusammenfassung |
| Climate models project increases in globally averaged atmospheric specific
humidity that are close to the Clausius–Clapeyron (CC) value of around 7% K−1
whilst projections for mean annual global precipitation (P) and
evaporation (E) are somewhat muted at around 2% K−1. Such global
projections are useful summaries but do not provide guidance at local (grid
box) scales where impacts occur. To bridge that gap in spatial scale,
previous research has shown that the "wet get wetter and dry get drier"
relation, Δ(P − E) ∝ P − E, follows CC scaling when the projected
changes are averaged over latitudinal zones. Much of the research on projected climate impacts
has been based on an implicit assumption that this CC relation also holds at
local (grid box) scales but this has not previously been examined. In this
paper we find that the simple latitudinal average CC scaling relation does
not hold at local (grid box) scales over either ocean or land. This means
that in terms of P − E, the climate models do not project that the "wet get
wetter and dry get drier" at the local scales that are relevant for
agricultural, ecological and hydrologic impacts. In an attempt to develop a
simple framework for local-scale analysis we found that the climate model
output shows a remarkably close relation to the long-standing Budyko
framework of catchment hydrology. We subsequently use the Budyko curve and
find that the local-scale changes in P − E projected by climate models are
dominated by changes in P while the changes in net irradiance at the surface
due to greenhouse forcing are small and only play a minor role in changing
the mean annual P − E in the climate model projections. To further understand
the apparently small changes in net irradiance we also examine projections
of key surface energy balance terms. In terms of global averages, we find
that the climate model projections are dominated by changes in only three
terms of the surface energy balance: (1) an increase in the incoming
long-wave irradiance, and the respective responses (2) in outgoing long-wave
irradiance and (3) in the evaporative flux, with the latter change being much
smaller than the former two terms and mostly restricted to the oceans. The
small fraction of the realised surface forcing that is partitioned into E
explains why the hydrologic sensitivity (2% K−1) is so much smaller
than CC scaling (7% K−1). Much public and scientific perception
about changes in the water cycle has been based on the notion that
temperature enhances E. That notion is partly true but has proved an
unfortunate starting point because it has led to misleading conclusions
about the impacts of climate change on the water cycle. A better general
understanding of the potential impacts of climate change on water
availability that are projected by climate models will surely be gained by
starting with the notion that the greater the enhancement of E, the less the
surface temperature increase (and vice versa). That latter notion is based
on the conservation of energy and is an underlying basis of climate model projections. |
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