Environmental Engineering Reference
In-Depth Information
that choices about stabilization targets depend upon judgments regarding
the degree of acceptable risk.
The likely range of cumulative carbon emissions corresponding to a
given warming level is estimated to lie between -30% to +40% of the best
estimate. This range is due mainly to uncertainties in the carbon cycle re-
sponse to emissions and the climate response to increased radiative forcing.
For a cumulative anthropogenic emission of 1,000 GtC, our best estimate
of the warming remains below 2°C, but there is an estimated 17% prob-
ability that the warming could exceed 2°C for more than 1,500 years. When
cumulative emissions are increased to 1,500 GtC, the best estimate of the
anthropogenic warming remains above 2°C for more than 3,500 years,
and the very likely upper end warming is still above 2.5°C for more than
10,000 years. Higher values cannot be excluded, implying additional risk
that cannot presently be quantified. On the other hand, at the lower end of
carbon-climate likely uncertainty range, there may be about a 17% chance
that warming could remain below 2°C even if as much as 1,700 GtC are
emitted. Figures S.3 and S.5 provide some scientific reasons why global
warming of a few degrees could be considered dangerous to some aspects
of nature and society, but the corresponding uncertainty ranges should be
emphasized here. For example, while the best estimate of a stabilization
target corresponding to a long-term warming of 2°C is 430 ppm, the likely
uncertainty range for this value spans from 380 ppm (below current ob-
served levels) to 540 ppm (almost a doubling of carbon dioxide relative to
pre-industrial times). {3.4, 6.1}
Many important aspects of climate change and its impacts are expected
to be approximately linear and gradual, slowly becoming larger and more
significant relative to climate variability as global warming increases.
This report highlights the importance of 21st century choices regarding
stabilization targets and how they can be expected to affect many aspects
of Earth's future. Progressively warmer temperatures are expected to slowly
lead to larger and more significant changes for impacts including wildfire
extent, decreases in yields of some (but not all) crops, streamflow changes,
decreased Arctic sea ice extent, increases in heavy rainfall occurrence, and
other factors presented. However, it should be noted that many climate
changes and impacts remain poorly understood at present. For example,
the record of past climates suggests that major changes such as dieback of
the Amazon forests or substantial changes in El Niño behavior can occur.
This report identifies some areas where recent science suggests reduced ef-
fects compared to earlier studies (including e.g., projected future changes in
