The global warming debateIt’s not about CO2, it’s about water!

Wetsus, 21 April 2011

Who am I?

• 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? ARGOThe 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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