Many complex processes shape our climate:
Based just on the physics of the amount of energy that CO2 absorbs and emits, a doubling of
atmospheric CO2 concentration from pre-industrial levels (up to about 560 ppm) would by itself
cause a global average temperature increase of about 1°C (1.8°F). In the overall climate system, however, things are more complex; warming leads to further effects (feedbacks) that either
amplify or diminish the initial warming. The most important feedbacks involve various form of water. A warmer atmosphere generally
contains more water vapour. Water vapour is a potent greenhouse gas, thus causing more
warming; its short lifetime in the atmosphere keeps its increase largely in step with warming. Thus, water vapour is treated as an amplifier, and not a driver, of climate change. Higher
temperatures in the polar regions melt sea ice and reduce seasonal snow exposing a darker
ocean and land surface that can absorb more heat, causing further warming. Another important
but uncertain feedback concerns changes in clouds. Warming and increases in water vapour
together may cause cloud cover to increase or decrease which can either amplify or dampen
temperature change depending on the changes in the horizontal extent, altitude, and
properties of clouds. The latest assessment of the science indicates that the overall net global
effect of cloud changes is likely to be to amplify warming. The ocean moderates climate change. The ocean is a huge heat reservoir, but it is difficult to
heat its full depth because warm water tends to stay near the surface. The rate at which heat is
transferred to the deep ocean is therefore slow; it varies from year to year and form decade to
decade, and it helps to determine the pace of warming at the surface. Observations of the sub- surface ocean are limited prior to about 1970, but since then, warming of the upper 700m
(2,300 feet) is readily apparent, and deeper warming is also clearly observed since about 1990. Surface temperatures and rainfall in most regions vary greatly from the global average because
of geographical location, in particular latitude and continental position. Both the average values
of temperature, rainfall, and their extremes (which generally have the largest impacts on
natural systems and human infrastructure), are also strongly affected by local patterns of winds. Estimating the effects of feedback processes, the pace of the warming, and regional climate
change requires the use of mathematical models of the atmosphere , ocean, land, and ice (the
cryosphere) built upon established law of physics and the latest understanding of the physical, chemical and biological processes affecting climate, and run on powerful computers. Models vary in their projections of how much additional warming to expect (depending on the type of
model and on assumptions used in stimulating certain climate processes, particularly cloud
formation and ocean mixing), but all such models agree that the overall net effect of feedbacks
is to amplify warming.
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 Nino
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
Learn more about other human causes of climate change:
In addition to emitting greenhouse gases, human activities have also altered Earth’s energy
balance through, for example:
Changes in land use.
Changes in the way people use land ____ example, for forests, farms, or cities __can lead to both warming and cooling effects locally by changing the
by changing how wet a region is.
Emissions of pollutants (other than greenhouse gases). Some industrial and
agricultural processes emit pollutants that produce aerosols (small droplets or particles
suspended in the atmosphere). Most aerosols cool Earth by sunlight back to space. Some aerosols also affect the formation of clouds, which can have a warming or cooling
effect depending on their type and location. Black carbon particles (or “soot”) produced
when fossil fuels or vegetation are burned generally have a warming effect because they
absorb incoming solar radiation. 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, 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 globally-averaged 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 21
st century [FIGURE B5]. 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.
