Aerosol-impacts as the climate models' feet of clay
Can it even become hotter than we feared of up to now?
The forecasts about the climate of tomorrow are not really optimistic. On June, 30th 2005 the
science magazine 'Nature' published an article which even amplifies the fear about a hot
future on our planet. The authors say, that today's dust in the air and its potential decrease in
the future is not sufficiently included in the present climate models. The more particles cool
the Earth at the moment, the hotter the future climate may become.
1. Climate scenarios of the Intergovernmental Panel on Climate Change IPCC: The potential
increase in the global temperature is calculated for diverse estimations. This does not only
include the climate factors themselves. Also developments in the world’s population, the
global economy, energy policy and new technologies have to be considered.
Please click the image in order to see the original graph.
Source: IPCC TAR – Summary for Policymakers
The larger the present-day aerosol cooling, the extremer the temperature increase in the
future.
The present climate scenarios have been developed in their basics from 1990 – 1992 and have
been improved afterwards (IPCC scenarios). The models are partially based on estimations of
climate changes in a far away past (paleoclimatology) as temperature saltations and
developments in the carbon dioxide content of the air between glacial times and interglacials,
i.e. warm periods in the Earth history. However, many factors in the climate models are
approved by simulating the changes in the last 200 years, because from this period we have
much more reliable data from modern meteorological measurements and scientific
observation.
In the recent years scientists learned a lot about the influence of aerosols. Human made
aerosols had always some influence on our climate in the last 200 years since industrialisation
began. Most likely they acted like a break preventing a too strong temperature increase while
the continuous increase in greenhouse gases fostered the process of global warming. In order
to protect our health we would like to stop applying this unwanted brake and begin to filter
more and more the particles coming from diverse processes in industry and transport. This
means however, the conditions of the future climate cannot be compared anymore with the
test conditions for our climate models.
2. Brown haze has been observed during the Indoex field campaign over the Indic Ocean.
Source: SCRIPPS Institution of Oceanography
The calculations of the authors in nature say: The aerosol cooling is like a protective coating
of our climate system. It did not react very much on the pinpricks exacerbated by the
continuous increase in greenhouse gas emissions. We do not know how thick this protective
layer really is. But the thicker it is today, the ore we have to worry that the future climate
reacts very sensitive on the increasing greenhouse gases, when this protective layer is more
and more reduced.
3. What is included in a climate model and how is it approved? All parameters which are put
in are subject to some uncertainty. Data from the climate of the former millennia are rare.
Therefore the comparison with younger measurement values is an important method in order
to validate the models.
Scheme: Elmar Uherek
Please click on the scheme for a higher resolution! (130 K)
Increasing likelihood of extreme dangers
The present models already estimate that the Earth of the year 2100 might be 2.3 – 4.9°C
warmer than it was in 1850. Considering the most recent knowledge on the impact of aerosols
it cannot be excluded that even a significantly stronger global warming could be possible. The
authors of the Nature article estimate that an average warming of 5-6°C is more likely than
the so far published values. 6°C is the temperature difference in the annual mean between
Berlin and Rome.
3. Temperature change simulated by a simple model according to Meinrat O. Andreae, Chris
D. Jones & Peter M. Cox (Nature, 30.06.2005). The red line is calculated for a present-day
aerosol forcing of Q = - 1,7 W/m2 (compare also the article: a model calculation). The blue
shading and the yellow line represent the range and central projection given in IPCC-TAR,
based on the same scenario. The blue line is calculated for the scenario without any presentday aerosol cooling.
Please click on the diagram for a full view! (20 K)
The climate models have a large range of uncertainty. Even larger is the uncertainty for the
estimations of the aerosol impact. Therefore we have to be aware that such numbers and
potential extreme values of a global warming of up to 10°C are based on theories which can
imply large mistakes. Therefore, some scientists do not agree and regard the estimations as
too extreme. The key statement of the calculations however is that the present knowledge
indicated that we can regard it as more likely now than 10 years before that the temperature
increase of the future will be in the extremer range of the IPCC scenarios. Since greenhouse
emissions increased in the recent 10 years and the projections of the International Energy
Agency do not foresee any change in the trend in the next 25 years, the gap becomes larger
between the estimates of the dangers made by scientists and the measures taken in our society.
How does the climate equation work?
A simple model calculation
The climate equation
We demonstrate the aerosol problem in a simple model.
In a climate model there are certain factors which drive (force) the climate system. These
forcing can be positive and lead to a warming. It can also be negative and lead to a cooling.
We call this forcing “delta Q”. If the driving factors change with time (for example from
industrialisation up to now) because the greenhouse gases accumulate or the average sun
radiation and cloudiness change, also the average temperature on the Earth (“delta T”)
changes. The change in temperature is the larger the more sensitive the climate system reacts
to changes in the forcing. This climate sensitivity is expressed by the Greek letter “Lambda”.
We can write now:
Temperature change = climate sensitivity x Antriebsfaktoren
with:
Temperature change in the considered time range
climate sensitivity
Change in the forcing during the considered time (in Watt per square meter)
What are the driving forces?
Climate research investigates which chemical and physical parameters stand behind this
driving forces. A major factor are the greenhouse gases: carbon dioxide, methane, ozone,
dinitrogenmonoxide, CFCs. Their fraction in the air has increased a lot since the
industrialisation. Greenhouse gases retain the energy of the infrared radiation close to the
Earth. This radiative forcing brings in about +2.4 W/m2. There are some other forcings which
can be either positive or negative but which have a less substantial impact. Finally aerosols
play a major role, either by direct reflection of the sunlight or via formation of clouds. Our
knowledge about the dimension of this effect is very poor. We can only guess that it may be
in the range of –1 to –2 W/m2.
We can now write a very simple equation about the sum of all driving factors:
How strong is the impact of the aerosol cooling on the climate sensitivity?
A simple calculation with whole numbers demonstrates that the climate sensitivity is strongly
dependent on the aerosol cooling. We assume that a temperature change of 2°C has been
observed. Furthermore the climate forcing coming from greenhouse gases and other factors
may sum up to 2 W/m2. Since we do not know exactly what the contribution of the aerosol
cooling is, we assume values of: 0 ; -1 ; -1.5 ; -1.75 W/m2. What is the resulting climate
sensitivity?
Antrieb
Treibhausgase
+ andere Antrieb
Aerosol
2+2021
2+2-112
2 + 2 - 1,5 0,5 4
2 + 2 - 1,75 0,25 8
We see that the climate sensitivity takes values of 1, 2, 4 and 8°C per W/m2. This is very
uncertain. Such a wide uncertainty is also given in the scientific climate calculations. Climate
models use for example sensitivities of 1.5 to 4.5°C per W/m2.
The graph on the left shows the result with a grey shaded uncertainty rang for a scientific
calculation.
Graph: Climate sensitivity estimation based on the ‘Nature’ article of Andreae, Jones and Cox
from 2005-06-30.
If the climate sensitivity is high ...
What will happen if we assume that the aerosol cooling is rather high (about 1.5 W/m2), but
will decrease in the near future, while in parallel the amount of greenhouse gases in the air is
increasing? We choose some numbers:
+ 2,5 1 1,5 4 6
+ 3 - 0,5 2,5 4 10
As we see the global temperature would increase significantly.
This example of a climate model calculation is strongly simplified. But it explains the basic
thoughts which are behind the worries of the scientists.
Particles: Impact on health and climate
Particles change the atmosphere
The light and energy of the sun have to pass a lot of barriers on their way through the
atmosphere. It is not only the water droplets of the clouds, which blur sometimes the sun.
Also fine particles (either liquid or solid) belong to these barriers in the air. We call them
aersols.
Dust and other particles have many natural origins: wind or plant emissions and in very
drastic cases volcano eruptions. Large amounts of dust come also from industries, car exhaust,
power plants, from human induced fires or other changes in the landscape. The anthropogenic
contribution to the particle load in the air is remarkable.
1. The impact of particles on the properties of our atmosphere becomes very obvious to us if
they reduce the visibility of the air like on this photo in the Smoky Mountains. (visibility
above: about 100 miles, below: about 20 miles)
© Oak Ridge National Laboratory
Particles burden our health and cool the Earth surface
Diseases caused by particles have been a day-to-day fate of many industrial workers or in
mining some decades ago. Today the conditions are better. However, in particular very fine
particles are still seen as a big danger for health. This caused the debate about ‘fine dust’. The
problem of sulphuric acid particles and acid rain is basically solved in Europe now by
filtering, but it is still a challenge in many developing countries. We could say: in order to
prevent damage from our environment and our own health we strive to reduce the human
made particle burden in the air as far as possible.
But particles play also a role in the energy budget of the Earth. Some of them like black soot
absorb light very well and warm the atmosphere. Others like sulphuric acid particles reflect
the sunlight and cool the Earth surface. We assume that the total impact of all particles effects
adds up to a significant cooling of the climate system which counteracts the greenhouse
effect.
2. Cut-out of the IPCC graph on factors forcing climate change. The impacts of greenhouse
gases (left column in green) and changes caused by aerosol are compared. For some of the
aerosol impacts we cannot give any value (column) but only an error bar due to the wide
uncertainty range.
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