Record global temperatures, melting polar ice caps and glaciers, shorter
winters, reduced snow cover, rising sea level, increased coastal flooding,
more frequent hurricanes, dramatic shifts in the distribution of wildlife,
vegetation and diseases, and changes in ocean currents.
These are just some of the consequences that have been associated with
changes in global climate caused by human activity – particularly the
pumping of so-called greenhouse gases into the atmosphere.
Noone disputes that, with a population exceeding 6 billion today and
likely to become 9 billion in 50 years, the potential of humanity to alter
our planet is now far greater than at any time during the previous 3,000
years of human history. However, the key questions remain. Is human
interference with Nature really causing long-term changes to our climate?
Just how realistic are the apocalyptic predictions?
Greenhouse Earth.
Our Earth is a fertile oasis in space. The blue planet so admired by
astronauts lies approximately 150 million km from the Sun. At this
distance, our water-covered world should be frozen, a globe covered in a
sheet of ice. On an airless Earth, the average surface temperature
would be 33 degrees lower: -18°C instead of the present average
temperature of +15°C. Only the atmosphere prevents Earth from freezing. In
particular, the presence of certain heat-trapping gases – the “greenhouse”
gases – provides our Earth with the mild, stable temperature that makes it
so hospitable for life.
The greenhouse gases, primarily water vapour and carbon dioxide, play a
crucial role in regulating the temperature of the Earth since they are
transparent to incoming solar radiation but they absorb some of the
infra-red (heat) radiation emitted by the warm surface. The result is an
increase in the temperature of the lower atmosphere.
Other trace gases such as methane, nitrous oxide, ozone and
chlorofluorocarbons (CFCs) also contribute to this global warming. Most of
these occur naturally, but they may also be generated by human activity.
The fear is that enhanced emissions of greenhouse gases from industry and
agriculture will also enhance the natural greenhouse effect.
There can be no disputing that concentrations of most greenhouse gases
have increased significantly in recent years. The atmospheric
concentration of carbon dioxide has risen by almost one third since 1750,
largely as a result of fossil fuel burning and land use changes such as
deforestation. Studies of air trapped in Antarctic ice cores indicate that
the current level is the highest for at least 420,000 years, and possibly
unprecedented for 20 million years.
It is a similar story for many of the other greenhouse gases. Methane
levels have trebled since 1750, nitrous oxide has risen by 17% and ozone
in the lower atmosphere (the troposphere) has increased by more than one
third. The situation is further complicated by the ability of certain
man-made pollutants, particularly oxides of nitrogen and organic
hydrocarbons, to have an indirect influence by generating ozone in the
lower atmosphere.
A Warmer World.
Scientists agree that global mean temperatures are half a degree Celsius
warmer than they were a century ago, that the amount of carbon dioxide in
the atmosphere has been increasing for two centuries, and that carbon
dioxide is the greenhouse gas that is most influential in warming the
Earth. It is also generally accepted that the 1990s were the warmest
decade and 1999 was the warmest year since records began in 1861.
However, despite this apparent link, the scientific community is unable to
agree on whether the warming is caused primarily by CO2 emissions, whether
it will continue, or whether it would be harmful if it did.
“One reason for this uncertainty is that the climate is always changing,”
said Richard Lindzen, professor of meteorology at Massachusetts Institute
of Technology. “Two centuries ago, much of the northern hemisphere was
emerging from a little ice age. During the Middle Ages, the same region
was in a warm period. Thirty years ago, we were concerned with global
cooling.”
Apart from the natural climatic variations, scientists are also struggling
to take into account the complex interactions that take place between the
atmosphere, oceans, land and biosphere. Only now, with the advent of
supercomputers and streams of data from Earth-orbiting satellites, are
global climate models becoming sufficiently realistic to make reasonably
accurate predictions.
However, even the most advanced computer models cannot forecast the future
with any precision, partly because of the complex interactions between the
Earth’s natural systems, and partly because they are often based on data
sets covering no more than a few decades.
This uncertainty is reflected in the latest Assessment Report issued by
the Intergovernmental Panel on Climate Change, which estimates that the
increase in the world’s average surface temperature over the period 1990
to 2100 could range from as low as 1.4°C to as high as 5.8°C.
Climate in the Balance
To keep our planet at an overall hospitable temperature, the Earth must
loose some heat energy into space. Earth’s outgoing energy has two
components: thermal radiation emitted by the Earth’s surface and
atmosphere, and solar radiation reflected back to deep space by the
oceans, lands, aerosols (tiny airborne particles) and clouds.
This balance between the incoming energy from the Sun and outgoing energy
back to space, which scientists refer to as the Earth’s “radiation
budget”, determines Earth’s temperature and climate. It is controlled by
both natural and human-induced changes, presenting scientists with a wide
range of possible scenarios to study.
For scientists to understand climate, they must also determine what drives
the changes within the Earth’s radiation balance. This is where satellite
data are playing an ever more influential role.
Earth orbiting spacecraft can obtain continuous measurements of many
climate indicators over the entire planet, something that was impossible
before the Space Age. By integrating data from space-borne instruments,
scientists can test and improve the accuracy of their global climate
models. This will eventually provide a new picture of the energy balance
from the top of the atmosphere all the way down to the surface of the
Earth.
Monitoring Our Changing Earth
While the available evidence suggests that human activity is having a
discernible influence on global climate, existing climate models still
reflect the great deal of uncertainty about the reasons for these changes
and their future impact on the habitability of our planet.
The mass of data returned by the new generation of environmental
satellites will help to clarify many of these uncertainties. The most
powerful and versatile of these is the European Space Agency’s ENVISAT,
the largest scientific satellite ever built in Europe.
ENVISAT, which is due for launch by the end of 2001, will carry 10
instruments that are designed to study our ever-changing planet. Most of
these instruments will have a direct impact on research into atmospheric
chemistry and global climate change.
One of the key roles of ENVISAT’s battery of instruments will be to
measure the concentrations of water vapour, trace gases and aerosols at
different levels in the atmosphere, something which is poorly quantified
at the present time.
This is important because large quantities of man-made pollutants,
including carbon, chlorine, ozone, nitrogen and sulphur compounds, have
been injected into the atmosphere over the last century. These not only
act as greenhouse gases, but they modify the chemistry of the upper
atmosphere – sometimes in unexpected ways.
The roles of aerosols and clouds in the global climate system also need to
be clarified. These can both absorb and scatter incoming solar radiation,
so influencing the amount of energy reaching the Earth’s surface.
Aerosols can also increase the rate at which solar energy is reflected
back into space by promotion of cloud formation. However, the extent to
which they modify Earth’s climate has been difficult to assess since
aerosols vary considerably in terms of size, shape and chemical
composition. Sensors on ENVISAT should improve our knowledge of the
origin, dynamics and eventual fate of aerosols.
Closer to Earth, ENVISAT will also monitor changes in surface conditions.
Since the oceans absorb at least half of the excess heat energy received
by the Earth and then transfer this energy from the tropics to the poles,
continuous recording of sea surface temperatures will be vital. ENVISAT
will also be able to monitor ocean currents and periodic climate-ocean
oscillations, such as the famous El NiÒo that affects the Pacific coastal
regions every few years.
ENVISAT will also measure ocean colour – the concentration of chlorophyll
in the upper layers of the ocean. This is important because it indicates
the abundance of microscopic plants and animals known as phytoplankton. By
monitoring the “blooming” of the oceans in spring, scientists can study
biological activity in the upper ocean and determine how much carbon is
likely to be stored.
“The uptake of CO2 by the oceans is one of the most challenging and
difficult to quantify,” said Professor David Llewellyn-Jones, Head of
Earth Observation Science at the University of Leicester, UK. “The oceans
dissolve CO2, which is then assimilated into the marine biological system
and returned to the carbon cycle. The biological productivity can be
inferred on a global scale from satellite observations of ocean colour.
The MERIS instrument on ENVISAT will give additional data about the
biological productivity of the oceans and its subsequent influence on CO2
uptake.”
Similarly, monitoring of vegetation on the Earth’s land surface will
provide improved estimates of how much carbon and energy is absorbed or
released into the atmosphere. This is vital, since there is currently
considerable uncertainty about the global carbon budget.
Finally, all-weather mapping from orbit of ice and land surfaces will give
vital clues about global climate trends. Studies of whether ice sheets and
glaciers are thinning, advancing or retreating, will provide scientists
with a sensitive indicator of climate change. Apart from the implications
for rising sea level and coastal flooding, such studies will also provide
information on the Earth’s albedo – the amount of solar radiation it
reflects into space.
“Radar echoes from the ice sheets enable us to estimate their elevations,”
said Dr. Seymour Laxon of University College, London. “By measuring
changes in the elevations of the ice sheets in Antarctica and Greenland,
we can determine whether they are growing or shrinking, and whether global
sea levels will be affected.”
“We are also collaborating with the Hadley Centre for Climate Change (in
the UK) to work the sea ice data into their models of climate change,” he
added. “This will help to improve the predictive capacity of these
models.”
Scientists unanimously agree that the development of increasingly
sophisticated computer models is essential to our understanding of future
climate changes.
“The only way to find out if the (climate) models are right is to get the
right observations,” said Professor Llewellyn-Jones. “The observations
tell us what’s there, then we set up a model to describe this behaviour
and make future predictions.”
“Finally, we use more observations to check our predictions,” he
explained. “This is the rationale behind all climate research – to be able
to make accurate predictions about what will happen in the future. This
would not be possible without observations from satellites such as ENVISAT
and ever more sophisticated computer models.”
Note to the Editors: all pictures relating to ENVISAT are available under
http://www.esa.int. The present information note is part of a series of
articles devoted to the ENVISAT programme and its applications.
ENVISAT N° 3
Annexe 1
GLOBAL CLIMATE CHANGE: BACKGROUND
The Rio Earth Summit
The United Nations Framework Convention on Climate Change (UNFCC) was
signed by an 181 governments – an overwhelming majority of the world’s
leaders – at the “Earth Summit” in Rio de Janeiro (Brazil) in 1992.
Article Two of the UNFCC, which came into force in 1994, obliged all
signatories to “achieve stabilisation of greenhouse gas concentrations in
the atmosphere at a level that would prevent dangerous anthropogenic
(human generated) interference with the climate system Ö within a time
frame sufficient to allow ecosystems to adapt naturally to climate change,
to ensure that food production is not threatened, and to enable economic
development to proceed in a sustainable manner.”
Developed nations – members of the OECD and the former Warsaw Pact – also
committed to limit their year 2000 emissions of greenhouse gases to 1990
levels.
The Kyoto Protocol.
Further targets for control of greenhouse gases were agreed at another
climate convention held in Kyoto, Japan, in December 1997. So far, 84
countries have signed the Protocol, but it has yet to be ratified.
The Kyoto Protocol required developed countries to reduce their annual
emissions of these gases by an average of 5.2% by 2008-2012, when compared
with 1990 levels. The EU as a whole undertook to achieve an 8% reduction,
with different targets set for individual member states.
A key compromise of the Kyoto Protocol was the recognition that developed
countries could meet their targets by transferring reductions in emissions
across national boundaries under so-called “flexibility mechanisms”.
These took three forms:
Emissions trading. A developed country that achieved larger reductions
than required was free to sell its “surplus” to another developed country
that failed to reach its target.
Joint implementation. A developed country would be able to fund projects
in other developed countries that would either reduce carbon dioxide
emissions (such as improving the efficiency of a power station) or enhance
carbon “sinks” (such as forest planting). The consequent reduction in
greenhouse gas emissions would be allocated to the country financing the
project.
Clean development mechanism. Developed countries could also fund
greenhouse gas reduction projects in less developed countries.
Post-Kyoto
The Kyoto Protocol was generally hailed as a vital first step in slowing
greenhouse warming. However, when leaders of the world’s nations gathered
in The Hague, the Netherlands, in November 2000, to discuss ratification
of the Protocol, the talks collapsed without any agreement being reached.
Similar disagreement surfaced in June 2001 when President George W. Bush
refused to ratify the Kyoto Protocol. One of the main stumbling blocks was
the Protocol’s omission of developing countries, potentially some of the
largest contributors to global warming in the decades ahead. The President
stated that those countries’ emissions of carbon dioxide will double
between 1990 and 2010 – an increase twice as large as the reductions the
United States would be obliged to make under Kyoto. Bush also argued that
Kyoto is unrealistic because many countries cannot meet their obligations
under the Protocol.
Without ratification by the United States, the world’s largest consumer of
energy and the major producer of greenhouse gases, the Kyoto Protocol is
unlikely to enter into force.
ENVISAT N° 3
Annexe 2
PERSPECTIVES ON GLOBAL CLIMATE CHANGE
Professor David Llewellyn-Jones, Head of Earth Observation Science, Space
Research Centre, Dept. Of Physics and Astronomy, University of Leicester,
UK
“What a satellite such as ENVISAT does is provide coverage, continuity and
consistency. We need all of these in order to study geophysical changes
which take place over long time periods. Often the change is quite small
and precise – for example, sea surface height and sea surface temperature
– two parameters that ENVISAT measures which are closely related to global
warming.
In the case of sea surface temperature, you need to look for a long time
to detect a definite trend. This is because there are annual variations
and other anomalies, such as El NiÒo, which occur periodically. This
manifests itself initially as an anomalous temperature rise of up to 4°C
in the eastern tropical Pacific. We are looking at a system that has a
certain amount of random variability and trying to detect substantive
changes which reveal themselves as a trend. We need accurate measurements
over long periods to characterise the natural variations.
The rate of change also causes problems. In attempting to predict global
warming, typical temperature trends expected for the sea surface or lower
atmosphere are in the region of 0.1°C per decade. Monthly variations often
show substantially greater rates of change than that, so we have to be
very careful with our analysis, so we have top be very careful with our
analysis in order to distinguish between natural variability and a genuine
warming trend.
We already have 10 years of data from ERS-1 and -2, but the 1990s were
marked by a highly irregular series of El NiÒo events – several small ones
and a massive one in 1997 – which affect our ability to extrapolate the
overall warming trend.
Latest analysis of this data set by Dr. Sean Lawrence at Leicester
suggests that there is a real warming trend of just under 0.1°C per
decade, but there are still considerable uncertainties about this
observation on account of natural variations. Therefore, it is essential
that we have the data from AATSR on ENVISAT, which will allow us to
continue and refine these measurements.
ENVISAT has additional instruments that can clarify the position over
climate change, in particular one that looks at ocean colour. Although the
planet’s radiation budget can be calculated, there are two really large
areas of uncertainty – clouds that reflect and block radiation, and the
carbon cycle, which involves the uptake of carbon dioxide through a number
of complex natural processes.
Of these processes, the uptake of CO2 by the oceans is one of the most
challenging and difficult to quantify. The oceans dissolve CO2, which is
then assimilated into the marine biological system and returned to the
carbon cycle. The biological productivity can be inferred on a global
scale from satellite observations of ocean colour. The MERIS instrument on
ENVISAT will give additional data about the biological productivity of the
oceans and its subsequent influence on CO2 uptake.
There are lots of processes of heat exchange between our planet and outer
space. We need to study each of these very carefully. ENVISAT will be a
big contributor on the observational side.
The only way to find out if the (climate) models are right is to get the
right observations. The observations tell us what’s there, then we set up
a model to describe this behaviour and make future predictions. Finally,
we use more observations to check our predictions. This is the rationale
behind all climate research – to be able to make accurate predictions
about what will happen in the future. This would not be possible without
observations from satellites such as ENVISAT and ever more sophisticated
computer models”.
Dr. Geoff Jenkins, Head of the Climate Prediction Centre in the Hadley
Centre for Climate Research, Bracknell, UK.
We will be using AATSR data (from ENVISAT) to monitor climate change. This
instrument will provide better information and global coverage of sea
surface temperature measurements, including continuity with ATSR-1 and -2
data, and additional information on cloud droplet and ice crystal sizes.
Two thirds of the world is sea. All the heat in the atmosphere diffuses
into the ocean on a continuous basis over hundreds or thousands of years.
However, the reaction time of the oceans can also be quite rapid. For
example, there was a rapid increase in sea surface temperatures in the
early 20th century, then it stabilised for a while before another rapid
increase since the 1970s. This has a direct impact on global temperatures
and climate. An obvious example is the influence of the warm Gulf Stream
on the climate of Western Europe.
An increase in the temperature of the ocean means that the water expands,
there is a rise in sea level and an increased potential for coastal
flooding.
Clouds are also very important influences on climate. They can only be
monitored by using satellites. We hope to use AATSR to study low level
water clouds and high level ice clouds. Cloud droplet and ice crystal
sizes are very important because they determine the amount of solar
radiation that is absorbed or radiated back into space. A small change in
water droplet size has a very big impact on climate. If the particle size
decreases, it means that more sunlight is scattered, leading to
atmospheric cooling. With an increase in particle size, the opposite
applies.
Dr. Seymour Laxon, Senior Lecturer at the Centre for Polar Observation and
Modelling, University College, London, UK.
There are three main reasons why sea ice studies are important for
research into global climate change.
First, the albedo. Sea ice reflects much more solar energy than the ocean
on which it sits. If some of the ice melts, then more open water is
exposed and this absorbs more radiation. This in turn melts more ice and
so it continues. This is an example of a positive feedback and is one of
the reasons why climate models predict the highest temperature increase in
the Arctic.
Second, the ice also acts as a insulating blanket, separating the very
cold atmosphere from the much warmer ocean beneath – the ocean temperature
may be typically around freezing point while the air temperature may be
-40 degrees Celsius. If the ice disappeared, then the blanket would
disappear and the temperature of the land areas around the Arctic would be
much higher than they are today.
Finally, the ice is responsible for transporting water around the Arctic
Ocean. Ice floes are the clouds of the ocean. When the ocean evaporates,
the water vapour eventually forms clouds elsewhere and it rains.
Similarly, sea ice draws fresh water up from the ocean, transports it into
the North Atlantic, then melts. This is particularly important for Europe
because there are some concerns that a change in the fresh water balance
of the North Atlantic might do something to the Gulf Stream – exactly what
might happen, no-one knows for sure.
We will use the radar altimeter on ENVISAT to measure the amount of sea
ice above the water level. Approximately one eighth of the ice is above
sea level, so we can estimate its thickness. We already have an eight-year
time series from Europe’s ERS satellites, the only satellite maps that so
far exist of sea ice thickness. We then compare the results with
measurements taken by submarines, in order to validate our estimates and
improve our interpretation of the large changes seen by the submarines.
The data from U.S. and European submarines suggest that there has been a
40% decrease in sea ice thickness since the 1950s, so it is obviously
important to find out whether this is a real trend. We need to have long
periods of continuous data rather than snapshots over limited areas. We
are looking for intrinsic trends in the data and natural cycles in the ice
cover. ENVISAT will help us to look for these long-term trends.
We are also collaborating with the Hadley Centre for Climate Change (in
the UK) to work the sea ice data into their models of climate change. This
will help to improve the predictive capacity of these models.
ENVISAT N° 3
Annexe 3
GLOBAL CLIMATE CHANGE: OTHER SOURCES OF INFORMATION
WEB SITES:
Intergovernmental Panel on Climate Change:
http://www.ipcc.ch/index.htm
UN Framework Convention on Climate Change:
http://www.unfccc.de/
World Climate Research Programme:
http://www.wmo.ch/web/wcrp/wcrp-home.html
Pew Centre on Global Climate Change:
http://www.pewclimate.org/
World Wildlife Fund Climate Change Campaign:
http://www.panda.org/climate/
The World Bank – Global Climate Change:
http://www-esd.worldbank.org/cc/
Center for the Study of Carbon Dioxide and Global Change, CO2 Science
Magazine:
http://www.co2science.org/
“Climate Change Science: An Analysis of Some Key Questions” – a Report to
the Bush Administration from the National Research Council of the National
Academy of Sciences:
http://www.nap.edu/catalog/10139.html?onpi_newsdoc060601
World Climate Report:
http://www.greeningearthsociety.org/climate/
UK Dept. of the Environment, Transport and the Regions:
http://www.environment.dtlr.gov.uk/ga/index.htm
Hadley Centre for Climate Prediction and Research:
http://www.met-office.gov.uk/research/hadleycentre/index.html
German Climate Research Centre:
http://www.dkrz.de/index-eng.html
Max Planck Institute for Meteorology:
http://www.mpimet.mpg.de/english/index.html
Laboratoire de MÈtÈorologie Dynamique du C.N.R.S.:
http://www.lmd.jussieu.fr/en/
The Cambridge-Conference Network (CCNet):
http://abob.libs.uga.edu/bobk/cccmenu.html
Italian Meteorology Laboratory
http://www.lamma.rete.toscana.it
ENVISAT N° 3
Annexe 4
GLOBAL CLIMATE CHANGE: GLOSSARY
Aerosols –
A collection of tiny airborne solid or liquid particles, with a typical
size between 0.01 and 10 microns (1 micron is one millionth of a metre).
They stay in the atmosphere for at least several hours. Aerosols may be of
either natural or anthropogenic origin. They may influence climate in two
ways: directly through scattering and absorbing radiation, and indirectly
through acting as condensation nuclei to assist cloud formation or
modifying the optical properties and lifetime of clouds.
Albedo –
The fraction of solar radiation reflected by a surface or object, often
expressed as a percentage. Clouds and snow-covered surfaces have a high
albedo; the albedo of soils ranges from high to low; vegetation and oceans
have a low albedo.
Anthropogenic –
Resulting from or produced by human beings.
Atmosphere –
The gaseous envelope surrounding the Earth. The dry atmosphere consists
almost entirely of nitrogen (78.1% by volume) and oxygen (20.9% by
volume), together with other trace gases such as argon (0.93%), carbon
dioxide (0.035%) and ozone. Amounts of water vapour are highly variable
but typically 1% by volume. The atmosphere also contains clouds and
aerosols.
Biomass –
The total mass of living organisms in a given area or volume. It includes
dead organic matter.
Biosphere –
The part of the Earth system comprising all living organisms – in the
atmosphere, land (terrestrial biosphere), or oceans (marine biosphere) –
as well as dead organic matter.
Carbon cycle –
The flow of carbon in various forms (e.g. carbon dioxide) through the
atmosphere, ocean terrestrial biosphere and lithosphere (Earth’s crust and
upper mantle).
Carbon dioxide (CO2) –
A naturally occurring gas, also a by-product of burning fossil fuels and
biomass, as well as land use changes and other industrial processes. The
principal anthropogenic greenhouse gas.
Climate –
The “average weather” as measured over a long period of time – typically
30 years.
Climate Change –
A statistically significant variation in either the mean state of the
climate or in its variability, persisting for an extended period of time,
typically decades or longer.
Climate model –
A numerical representation of the climate system which attempts to include
the physical, chemical and biological properties of its components, their
interactions and feedback processes, while accounting for all or some of
its known properties. Such models are used not only to study and simulate
the climate, but for monthly, seasonal and long term predictions.
Cryosphere –
The component of the climate system consisting of all snow, ice and
permafrost on or beneath the surface of the Earth.
Ecosystem –
A system of interacting, living organisms together with their physical
environment. They may range from the very small scale (e.g. a pond) to the
entire Earth.
El NiÒo –
A warm water current that periodically flows along the coast of Ecuador
and Peru, disrupting the local climate and fishery. This oceanic event is
associated with the Southern Oscillation – a fluctuation in the pattern of
surface air pressure and circulation in the Indian and Pacific Oceans. It
has climatic effects throughout the Pacific and in many other parts of the
world. The opposite of an El NiÒo event is called La NiÒa.
Energy balance –
A long term balance, averaged over the entire globe, between incoming
solar radiation and outgoing radiation – reflected solar radiation and
infrared radiation emitted by the climate system. Human induced or natural
changes to these inputs and outputs will upset the balance and may lead to
global warming or cooling.
Fossil fuels –
Fossil carbon deposits e.g. coal, oil, and gas that are burned to produce
energy. Their combustion results in emissions of carbon dioxide.
Greenhouse effect-
The entrapment of heat by various greenhouse gases in the atmosphere. This
occurs through the absorption of infrared radiation emitted by the Earth’s
surface, by the atmosphere itself, and by clouds. The natural greenhouse
effect can be enhanced by an increase in the concentration of greenhouse
gases through human activity.
Greenhouse gases –
The gaseous constituents of the atmosphere, both natural and anthropogenic
that absorb and emit infrared radiation (heat). The most important of
these are water vapour, carbon dioxide,
