Transcript Dia 1

The global warming debate
It’s not about CO2, it’s about water!
Wetsus, 21 April 2011
Who am I?
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Marcel Crok (1971)
University degree in chemistry (Leiden, 1995)
Science journalist since 1997
Worked for De Ingenieur (2000-2002) and for
Natuurwetenschap & Techniek (2003-2008)
• Published long article about the infamous
hockey stick graph in 2005
• 2008-2010 Worked fulltime on the book De
staat van het klimaat
Ocean versus
atmosphere
The energy content
of the oceans is a
thousand times
larger than that of
the atmosphere!
So tiny changes in
the oceans can
have a huge effect
on the atmosphere
How do we diagnose global
warming so far?
Is there a warm bias in the land
measurements? Yes!
klotzbach 2009
Is global warming accelerating?
No! It’s decelerating!
Is global warming accelerating?
No! It’s decelerating!
Is the sea level rise accelerating?
No!
Sea level rise
• IPCC wrote in 2007: “Whether the faster rate
[of sea level rise] for 1993 to 2003 reflects
decadal variability or an increase in the longer
term trend is unclear.”
• The answer to this becomes clearer every
year: it indeed seems to be decadal variability
• The rate during the most recent 10-yr period
is 2.32 mm/yr; This is not much above the
20th century average rate of 1.8mm/yr
What about the oceans? ARGO
The most relevant place to diagnose
global warming
Some basics: Forcings in AR4
So there should be warming, but…
• The IPCC forcings are based on differences
between 1750 and now. So this figure is not
showing the current net forcing
• Based on Ocean Heat Content data between
1993-2008 the best estimate of the current
net forcing is around 0,6 W/m2
• However, the latest ARGO data suggest a net
forcing of only 0,2 W/m2 (or even less)
• So ARGO data are wrong or our understanding
of forcings and feedbacks are wrong
Why do climate models warm up?
Forcings and feedbacks
• Theoretical warming for 2x CO2 is 1 degree
Celsius
• With three degrees warming – the best
estimate of IPCC for 2x CO2 – most of the
warming is coming from positive feedbacks
like water vapor, clouds and albedo changes
• However is the real climate behaving as the
models show?
• The global warming debate is about feedbacks
and the reliability of models, not about CO2
Water vapor feedback
• Most climate scientists feel ‘certain’ about a
positive water vapor feedback
• It seems so obvious: warmer air can hold
more water vapor and water vapor is a
powerful greenhouse gas
• However…
There are two competing processes
And the second process –
precipitation – we don’t
understand very well
Observational evidence?
Source: Trenberth 2005
IPCC Summary for Policy Makers
• “The average atmospheric water vapour
content has increased since at least the 1980s
over land and ocean as well as in the upper
troposphere. The increase is broadly
consistent with the extra water vapour that
warmer air can hold.”
• However the time series is short (1988-2004)
and the trend seems influenced by the strong
El Niño of 1998
How does the period 1988-2010
look like?
• Trenberth 2011 didn’t update his earlier graph
• However he stated that water vapor trends
are strongly related to Sea Surface
Temperatures
• How did Sea Surface temperatures evolve in
the last decade?
Sea Surface Temperature trend
Corresponding water vapor data
What about the Nasa dataset?
• NASA Water Vapor Dataset; NVAP is unique in
that it covers global land and ocean by
combining a variety of input sources
• Tom Vonder Haar by email this week: All we
can say at present is that the preliminary
NVAP data, according to the Null Hypothesis,
cannot disprove a trend in global water vapor
either positive or negative
Conclusions water vapor feedback
• Although a positive water vapor feedback is
intuitively quite plausible, there is little
observational evidence that this feedback has
been operational in the last decades
• The cloud feedback is even more uncertain,
which is also admitted by IPCC
• So there is no observational evidence yet for a
total net positive feedback. This could partly
explain why global warming is happening at a
slower rate than expected by the models
Nature: More extreme
precipitation caused by CO2
• Andrew Weaver: “We should continue to expect
increased flooding associated with increased extreme
precipitation because of increasing atmospheric
greenhouse gas. And we have no one to blame but
ourselves.”
• Judith Curry: “I find this kind of analysis totally
unconvincing, and it does not recognize the role of
natural internal variability such as the Arctic
Oscillation, La Nina, etc in producing floods. None of
the recent floods are extreme in historical context.”
First: are there global trends in
rainfall on land? Mwah
Global Precipitation Climatology Project
• One of the major goals of GPCP is to develop a
more complete understanding of the spatial
and temporal patterns of global precipitation.
Data from over 6,000 rain gauge stations, and
satellite geostationary and low-orbit infrared,
passive microwave, and sounding
observations have been merged to estimate
monthly rainfall on a 2.5-degree global grid
from 1979 to the present.
Global trends in rainfall since 1979
Extreme precipitation in The
Netherlands since 1950
Extreme precipitation in De Bilt
since 1906
Extreme precipitation in Europa
So is there really a trend in
extreme precipitation?
• No! Concluded the Greek hydrologist Demetris
Koutsoyiannis recently at the EGU
• All 3070 stations fulfilling the criteria set are
examined for trends. For the entire period, 1731
stations show positive slope and 1339 negative
slope. For the most recent 40 years, 1494 exhibit
positive slope and 1576 negative slope. “Strong”
negative trends beyond one sample standard
deviation have become more frequent compared to
those the entire period, while “strong” positive
trends have become slightly less frequent.
No clear trends
in extreme
precipitation!
Paper: “Dreary State Of
Precipitation In Global Models”
• these models produce precipitation
approximately twice as often as that observed
and make rainfall far too lightly
• This implies little skill in precipitation
calculated at individual grid points, and thus
applications involving downscaling of grid
point precipitation to yet even finer‐scale
resolution has little foundation and relevance
to the real Earth system.
Are models able to simulate rainfall
back in time?
• Two papers by Koutsoyiannis addressed this
question. In the second paper he looked at
temperature and rainfall at 55 grid boxes
• On annual and climatic time scales (30 year)
there was no correlation at all between the
models and reality!
• At EGU 2011: models underestimate extreme
rainfall up to a factor ten!
Are models ready for prime time?
• EDITORIAL, Are climate models “ready for
prime time” in water resources management
applications, or is more research needed?
Zbigniew W. Kundzewicz & Eugene Z. Stakhiv,
Hydrological Sciences Journal
• What does this all mean? The future is
unknown. We cannot say much about future
trends in rainfall and extreme rainfall
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