Re: [asa] Level of certainty in science

From: PvM <>
Date: Mon Feb 05 2007 - 23:58:23 EST

This seems to be a good opportunity to address some of the issues that
were raised in the global warming threads.

First of all, I ran across the following excellent resources

First of all a forum at NERC where scientists defend the science
behind global warming while addressing questions and issues raised by

<quote>Scientific evidence demonstrates clearly that human activity is
changing the planet's climate. But there are still sceptics who
dispute the data and its interpretation. If you don't believe the
science, please tell us why and we'll do our best to respond to your

Then there is the Stern review

<quote>The Stern Review on the Economics of Climate Change was
launched on 30 October 2006. The Review, led by Prof Sir Nicholas
Stern (Head of the Government Economic Service and former World Bank
Chief Economist), is the most comprehensive ever undertaken on the
economics of climate change.

Climate change research and projections undertaken by the Met Office
Hadley Centre, provided the core scientific input to the Review.

The Review focuses on the impacts and risks arising from climate
change, the costs and opportunities associated with tackling it, and
the national and international policy challenge of moving to a
low-carbon global economy.

The Stern Review shows that there are huge amounts of money at stake
and that there is real potential in terms of what relevant climate
research can do.

If you would like to discuss how we may can add value to your climate
change business plans and general operations please email or contact our Customer Centre
and we will be able to provide you with intimate climate knowledge and
its impacts on your business.</quote>

And the in depth resources from the Met Office's Hadley Center

With in depth publications outlining the many issues related to climate change

Having shared these resources, i would like to present my position on
global warming which is that over the last 40 years, the climate has
experienced a significant increase in temperature linked to human
caused increases in CO2.

As long as we remember that this is the very simple and limited
conclusion, we can explore some of the skepticisms raised against this
position. Some of the skeptics argue that CO2 has not always been the
cause of higher temperatures, or that natural CO2 sources and sinks
dwarf the small contributions by humans. So it is important that one
appreciates what is mean by climate change and what is not.

For instance global warming does not mean that the temperatures all
over the world should increase, in fact, climate models show that in
some cases temperatures can DECREASE significantly. Global warming
means averaged over the globe.

Second of all, CO2 is but one of the many relevant aspects to what
drives our climate, other variables are the solar input, cloud cover,
aerosols, other greenhouse gasses, the interactions between the
atmosphere and the oceans. What however science has shown is that in
order to explain the recent increases in temperature one has to add
the anthropogenic forcings and that when they are added, the resulting
temperatures match the observations quite well. In other words, the
science is based on 1) a model of the greenhouse effect 2) an
understanding of the role of CO2 in the greenhouse effect 3) an
understanding of the climate response to increases in CO2 and 4) an
understanding of the sources of this increase in CO2.

Now I can imagine that there will be those who remain skeptical about
the whole issue and before they present yet another variation of an
'objection' such as natural CO2 dwarves anthropogenic CO2, solar
forcings, the fact that CO2 lags the warming periods by a few hundred
years, that the antarctic is cooling in some places and snow is
increasing and so on and pause to ask oneself the simple question: Do
scientists not know about this? Have they been ignoring it? In most
instances the answer is straightforward: scientists have been aware of
these issues and they are often explained in straightforward manner.

Another objection to the use of computer models has been that the
system is chaotic and that the various mechanisms that govern the
models are not fully known. Again, as Lorenz argued, weather
predictions can be made with a certain level of certainty for only a
limited days in the future due to the chaotic nature. Which means that
weather models and climate models work in very different manner. While
both are based on fundamental laws of physics, thermodynamics,
hydrodynamics, climate models use probabilistic approaches. Remember
that chaotic does not mean random but rather that small changes may
lead to larger changes when solutions approach a different limit
cycle. Which is why climate models are not going to predict what year
will have an el nino, or what the weather is going to be today, but
rather what the global weather may look like in the future. With this,
estimates of variability are attached based on many a probabilistic
approach rather than a more deterministic approach.
So how do we know that these models work? Well, simple: by using known
data from the past and see how well these models work. When some
models work better than others, determine what causes them to fail. In
the end, by running a variety of climate models, with a probabilistic
approach, climate predictions can in fact be made with a certain
accuracy, despite the fact that the weather itself is 'chaotic' in
nature. By comparing hindcasts and forecasts with known data, such
models can be validated.
When it comes to ice dynamics, the model runs showed that they models
typically underestimated the melting of ice when compared with actual
data and this is one of the reasons why the ice-sheet melt were
excluded from AR4. This does however not mean that most models all
show a significant reduction in ice cover, but the variability seems
too large to be useful for predictions.

So it's the combination of a predictive model of global warming,
combined with observations of CO2 increase, and the fact that models
require these additional forcings to match the observations which
explains why the IPCC has made its predictions of global warming as it
has, and why its predictions have been strengthened in the last 6
years or so.

So, it is important to ask yourself this question whenever you run
across something that proclaims to show that there is no global
warming or that global warming is not mostly human caused: do these
findings improve the predictions based on models in reconstructing the
temperature trends? For instance, if solar cosmic rays where to be of
similar strength to the CO2 signature, then it is obvious that the
warming trend would be too large. Since we know the CO2 signature and
understand its impact on the temperature quite well, there needs to be
significant offset from another source which reduces the temperature
to the same extent as the CO2 forcing. The question now becomes; what
would such a negative forcing look like? In addition, you may want to
ask the question: How solid is the science behind these factors and
are scientists totally unaware of these factors?
In most cases you will come to realize that science has in fact
addressed these factors. Remember that the AR4 report will have
reviewed most of the recent arguments against global warming and that
in a peer review of thousands these arguments were found to be
Note that this does not mean that science cannot be wrong. It's just
that with more and more data coming in, science is more and more
certain of the findings that humans are a significant contributor to
global warming.
And that's because that's what the science, the data, the models all
strongly suggest.
Can science be wrong? Sure. Has science been wrong before? Sure. Will
science be wrong again? Possibly but less and less likely. Remember
how people objected to the Ozone argument? And how science showed how
there was indeed a strong causal link between human causes and the
widening and deepening of the ozone layer?
There is always time for a healthy amount of skepticism, or science
would cease to exist. What this means is however that such a
skepticism should come with an obligation to consider some of the
basic questions such as 1) does science know about this? 2) how does
science explain this?

Remember that it is not sufficient to show that there are other
components or factors to global warming, but rather that any
significant forcing needs to have another negative forcing to offset
the CO2 contributions to global warming. In other words if I propose
that X explains 80% of the global warming then I have to explain what
happened to about 80% of the warming caused by CO2. Otherwise, we have
a balance problem and heating predictions would be too large. Now it
may very well be the case that the CO2 contribution is (magically?)
counteracted by another component and that global warming has another
cause, but such an explanation requires more than showing that there
are additional components which may be able to play a role in the
heating. One has to show that addition of such factors improves the
predictions and models. after all the models are all based on
fundamental principles of physics.

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Received on Mon Feb 5 23:58:53 2007

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