Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 306

carbon sinks, keeping emission rates the same will not lead to stabilization of car-
bon dioxide. Emissions reductions larger than about 80%, relative to whatever peak
global emissions rate may be reached, are required to approximately stabilize carbon
concentrations for a century or so at any chosen target level. It also concluded that
certain levels of warming associated with carbon dioxide emissions could lock the
Earth and many future generations of humans into very large impacts; similarly, some
targets could avoid such changes. It made clear the importance of our 21st century
choices regarding long-term climate stabilization. Among the biological and human
impacts it noted that nearly 40% of global maize production occurs in the USA, much
of which is exported. It concluded that the most robust studies, based on analysis of
thousands of weather station and harvest statistics for rain-fed maize ( 80% of US

production), suggest a roughly 7% yield loss per ◦ C of local warming. It also warned
that changes in climate and carbon dioxide beyond 2100 will probably be sufficient
to cause large-scale shifts in natural ecosystems (see section 6.1.5) and that the Initial
Eocene or Palaeocene-Eocene Thermal Maximum (IETM or PETM) provides one of
the most worrying indicators of Earth system sensitivity and is the closest analogue
to what might happen to the carbon cycle in a climate substantially warmer than the
present (see section 3.3.9). The report noted that that the Earth system is subject
to feedbacks amplifying polar warming, which are not adequately represented in
first-decade 21st-century computer models. It also considered very long-term (in the
human sense) consequences, and considered that over the coming millennia some
impacts of climate change may settle into new patterns of climate variability with the
successful implementation of stabilization policies that cap cumulative emissions and
therefore limit increases in global mean temperature. They thought that it is possible
that societies could become accustomed to these new environments. Even so, that
future world would be different from today, but new conditions could become routine
to people living on Earth one or two thousand years from now. Other impacts, though,
could continue for many centuries past the date of temperature stabilization some
time following our ceasing to add greenhouse gas to the atmosphere.
So, what will the North American climate be like towards the end of the 21st
century? Readers are urged to seek out the climate change forecasts for the USA in
the USGCRP 2009 report and compare them with the IPCC's 2007 forecasts (and,
when published, the 2013 IPCC forecasts), not to mention the National Research
Council's 2010 report. Nonetheless, readers might like at least a broad-brush picture
of what may be expected and so the maps in Figure 6.6 very roughly portray some of
the salient points of these projections.
Turning to Canada, we have already covered some of the issues affecting natural
systems in the course of this section's largely-US focused discussion. However,
Canada's high-latitude environment deserves special attention, not withstanding that
Arctic amplification (see section 5.3.1) is seeing greater climate change in those
latitudes. In November the 2005 the Arctic Climate Impact Assessment ,or AICA
(ACIA, 2005), prepared by more than 250 scientists for the eight-nation Arctic
Council, affirmed predictions that anthropogenic warming would hit the Arctic earlier
and more significantly than other parts of the planet. It reiterated the IPCC (2001b)
scientific consensus that human emissions of greenhouse gases are causing much
of the warming observed in recent decades. However, the President George Bush

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