The greening of the North: real, and caused by climate change

Some
years ago Ranga Myneni and his colleagues at Boston University published
a paper in Nature apparently showing a "greening trend" in
the boreal regions. The finding was based on satellite observations,
and many scientists were sceptical about it because of problems of instrumental
"drift" that could possibly make any observed long-term trend
unreliable. The Boston team, however, persisted in their efforts to
re-analyze the data, taking into account all possible anomalies. The
trend would not go away, and it has become clearer with time as new
data have come in.

A possible
cause was always obvious: weather station records from the North have
reported a steady warming (by about 0.4 degree per decade), which is
now generally attributed to the increasing greenhouse effect. Warming
in the North might be expected to produce "greening". However,
it was not trivial to connect the weather station data to the satellite
data to find out whether there was a quantitative correlation between
the two trends. Although computer-based numerical models of the physical
climate have been used for three decades to analyze the possible consequences
of increasing greenhouse gas concentrations, comparable global models
of ecosystem dynamics are a much more recent development.

Now, a
major activity of the Global Ecology research group at MPI-BGC, under
the leadership of Prof. Colin Prentice, is the ongoing development of
a leading global terrestrial ecosystem model. The model is called LPJ,
after the three research groups (led by Prof. Colin Prentice at MPI-BGC,
Prof. Wolfgang Cramer at PIK; and Prof. Martin Sykes at Lund University)
that participate in the model development consortium. The basic idea
of the model is to integrate current knowledge in separate fields (plant
physiology and biophysics, terrestrial ecology and hydrology), to simulate
the interaction of processes with different time constants (minutes
to years), and to use all possible sources of information – including
satellite observations – to evaluate and refine the model. The new results,
published in Science on 31 May 2002, are the product of a co-operation
between the LPJ and Boston teams – spearheaded by Dr Wolfgang Lucht,
a young scientist at PIK with close links to the remote sensing community
– in which month by month climate observations from across the entire
boreal zone were used to drive the LPJ model. It was found that the
expected changes in leaf area index during each growing season and (most
importantly) from season to season, based on climate data, fitted remarkably
well to the satellite data. Two decades of satellite observations showed
that spring has advanced by about a week, and summer maximum leaf cover
has increased as well. The model results showed that these trends correspond
quantitatively to what should be expected, based on the warming that
has occurred. The only plausible explanation for this agreement between
two fully independent sources of information is that the remotely sensed
trend is real, and that it was caused by the changing climate.

With a
model such as LPJ, it is also possible to perform sensitivity tests
to isolate the key factor causing a given effect. It was found that
the trend was caused entirely by temperature. It was not caused by physiological
effects of increasing carbon dioxide concentration on plant growth (which
are likely to be much more important in warm climates than in cold climates),
or by the increasing rain- and snowfall that has accompanied the increase
in temperature.

One major
event temporarily interrupted the trend: the 1991 eruption of Mount
Pinatubo in the Philippines, whose fallout of volcanic aerosol produced
a 0.5 to 1 degree average cooling – as well as spectacular sunsets!
– in northern high latitudes during 1992-3. Again, satellite data had
already shown an interruption to the greening trend. And once again
many scientists were sceptical because the volcanic aerosol itself affects
the performance of the space-borne instruments. But in this case too
the LPJ model results showed the same thing as the satellite observations
– a short-lived dip in the trend, which was promptly resumed after the
volcanic dust had settled.

The Pinatubo
eruption had another effect too, which greatly surprised carbon cycle
scientists when it occurred: The rate of increase of carbon dioxide
in the atmosphere suddenly slowed down for two years. A full explanation
for this (which probably involves temperate and tropical regions as
well) has not yet been found. It was possible to engage the LSCE team,
which has been using advanced mathematical tools to analyze the small
variations in carbon dioxide concentration that are observed between
different measurement stations across the globe, in order to infer the
changing patterns of regional sources and sinks of carbon dioxide on
land and at sea. Their results showed that during the "Pinatubo
years" the land at high latitudes was taking up a great deal more
carbon dioxide than usual.

The LPJ
model results showed exactly the same thing. Even though the growing
season was shortened and photosynthesis reduced, the cooler growing
season meant slower decomposition of organic matter in the soil. This
was the dominant effect of cooling for the net carbon exchange of the
land in high latitudes, and it contributed to the slowing of the carbon
dioxide growth rate at a global scale.

These results
represent a "coming of age" for ecosystem modelling. They
clearly demonstrate the value of collaboration between observational
and theoretical communities (which has often been deficient in global
change research). Much ecosystem modelling up to now has been aimed
at forecasting the impacts of future climate change, but in this aspect
models show large differences. Thus, models need better evaluation,
and the only way to do this is by making systematic comparisons between
model predictions and observations referring to the present and past.

The results
have implications beyond academia. We have shown that the biosphere
is indeed changing, due to a climate change that most likely is a direct
consequence of human activities (principally in the industrialized countries).
The warming and "greening" of high latitudes, if they continue,
will have mixed effects from the point of view of the societies involved.
The potential for forestry and agriculture in northern Canada and Siberia
will improve, but Arctic ecosystems and species, and the indigenous
cultures that depend on them, will be threatened by the invasion of
plant and animal species from the south.