Human activities are changing the
climate.
Rigorous
analysis of all data and lines of evidence shows that most of the observed
global warming over the past 50 years or so cannot be explained by natural
causes and instead requires a significant role for the influence of human
activities.
In order to
discern the human influence on climate, scientists must consider many natural
variations that affect temperature, precipitation, and other aspects of climate
from local to global scale, on timescales from days to decades and longer. One
natural variation is the El NiƱo Southern Oscillation (ENSO), an irregular
alternation between warming and cooling (lasting about two to seven years) in
the equatorial Pacific Ocean that causes significant year-to-year regional and
global shifts in temperature and rainfall patterns. Volcanic eruptions also
alter climate, in part increasing the amount of small (aerosol) particles in
the stratosphere that reflect or absorb sunlight, leading to a short-term
surface cooling lasting typically about two to three years. Over hundreds of
thousands of years, slow, recurring variations in Earth’s orbit around the Sun,
which alter the distribution of solar energy received by Earth, have been
enough to trigger the ice age cycles of the past 800,000 years.
Fingerprinting
is a powerful way of studying the causes of climate change. Different
influences on climate lead to different patterns seen in climate records. This
becomes obvious when scientists probe beyond changes in the average temperature
of the planet and look more closely at geographical and temporal patterns of
climate change. For example, an increase in the Sun’s energy output will lead
to a very different pattern of temperature change (across Earth’s surface and
vertically in the atmosphere) compared to that induced by an increase in CO2
concentration. Observed atmospheric temperature changes show a fingerprint much
closer to that of a long-term CO2 increase than to that of a fluctuating Sun
alone. Scientists routinely test whether purely natural changes in the Sun,
volcanic activity, or internal climate variability could plausibly explain the
patterns of change they have observed in many different aspects of the climate
system. These analyses have shown that the observed climate changes of the past
several decades cannot be explained just by natural factors.
How will climate change in the
future?
Scientists
have made major advances in the observations, theory, and modelling of Earth’s
climate system, and these advances have enabled them to project future climate
change with increasing confidence. Nevertheless, several major issues make it
impossible to give precise estimates of how global or regional temperature
trends will evolve decade by decade into the future. Firstly, we cannot predict
how much CO2 human activities will emit, as this depends on factors such as how
the global economy develops and how society’s production and consumption of
energy changes in the coming decades. Secondly, with current understanding of
the complexities of how climate feedbacks operate, there is a range of possible
outcomes, even for a particular scenario of CO2 emissions. Finally, over
timescales of a decade or so, natural variability can modulate the effects of
an underlying trend in temperature. Taken together, all model projections
indicate that Earth will continue to warm considerably more over the next few
decades to centuries. If there were no technological or policy changes to
reduce emission trends from their current trajectory, then further
globallyaveraged warming of 2.6 to 4.8 °C (4.7 to 8.6 °F) in addition to that
which has already occurred would be expected during the 21st century.
Projecting what those ranges will mean for the climate experienced at any
particular location is a challenging scientific problem, but estimates are
continuing to improve as regional and local-scale models advance.
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