Present climate and biological change - Climate Change: Biological and Human Aspects - page 235

field observations and the palaeo record of Holocene shrub expansion). However, it is
thought that the increase in shrubs only accounts for 2% of recent warming. A further
3% may be attributable to increased tree cover, especially that of the white spruce
( Picea glauca ) whose cover has increased, decreasing the treeless area by 2.3%.
Although this makes only 5.3% of Arctic Alaskan summer warming accountable
to changes in vegetation cover (and the increased snow-free season accountable for
much of the rest) the researchers warn that the region is on the cusp with a number
of thresholds about to be crossed. The conversion of Arctic Alaskan tundra to spruce
forest (very much a possibility by the early 22nd century) has never before occurred
during previous Holocene warm intervals; that is, the slightly warmer episodes over
the past 11 700 years since the end of the last glacial. Given that Alaska would
have been covered in ice during the glacial, it means that it could not have been
covered by forest for more than 100 000 years since the brief height of the last
interglacial. In addition to ice-water-related thresholds, as shrubs become established
so the nitrogen cycle is enhanced, which in turn encourages further vegetation. This
is another positive feedback serving to accelerate matters. In short, the research
suggests that the already anomalous increased warming of Arctic Alaska, as part of
polar amplification, is likely to become even more pronounced in the future (Chapin
et al., 2005). Environmental change thresholds (critical transitions) are currently a
priority area of climate research and, as the abovementioned research suggests, are
central to high-latitude warming over and above those that computer models have
predicted to date.
Including biology and landscape change factors in computer models is now begin-
ning to show that there are greater differences between the various of the IPCC's
2000 SRESs for the 21st century (IPCC, 2000). Even between scenarios with sim-
ilar carbon dioxide at a given time there can be differences in temperature due to
the different biological landscape in different scenarios. The IPCC's A2 storyline
of these SRESs is one of more regional resource use and self-sufficiency with a
slightly slower economic growth. This is in contrast to the B1 storyline with the
same population up to the mid-21st century (but which declines thereafter), and
again increased sustainability, but through more global solutions and without extra
climate initiatives. In 2005 Johannes Feddema and colleagues showed that incor-
porating differences in the biological landscape from these two scenarios resulted
in somewhat different temperature profiles compared to that portrayed in the IPCC
2001 assessment. The agricultural expansion in the A2 scenario appeared to result
in significant additional warming over the Amazon and cooling of the upper air
column as well as nearby oceans. These effects in turn influenced the Hadley and
monsoon circulations and so modified some climate regimes outside of the tropics.
Meanwhile the model of Feddema and colleagues showed that agricultural expansion
in mid latitudes produced cooling and decreases in the mean daily temperature over
many areas (Feddema et al., 2005). Although it is early days for such biological land-
scape inclusions in global models, they will undoubtedly soon become an increas-
ingly routine, if not fundamentally intrinsic, addition to global climate computer
simulations.
So, not only do models need to accommodate more biological components and in
considerable detail, modellers need to keep abreast of biological research outputs. For