The predicted effects of global warming on the environment and for human life
are numerous and varied. It is generally difficult to attribute
specific natural phenomena to long-term causes, but some effects of
recent climate change may already be occurring. The 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) includes a summary of the expected effects, which I will discuss in a while.

An Overview

But, for the time being, let's get some insight on the topic: Global Warming. Projected climate changes due to global warming have the potential to lead to future large-scale and possibly irreversible effects at continental and global scales.

The
probability of one or more of these changes occurring is likely to
increase with the rate, magnitude, and duration of climate change.
Additionally, the United States National Academy of Sciences has warned, "Greenhouse warming
and other human alterations of the earth system may increase the
possibility of large, abrupt, and unwelcome regional or global climatic
events. Future abrupt changes cannot be predicted with confidence, and
climate surprises are to be expected." Most of the consequences of
global warming would result from one of three physical changes: sea
level rise, higher local temperatures, and changes in rainfall
patterns.

Effects of Global Warming:

1. Effects on weather

Global warming is responsible in part for some trends in natural disasters such as extreme weather. Increasing temperature is likely to lead to increasing precipitation but the effects on storms are less clear. Extratropical storms partly depend on the temperature gradient, which is predicted to weaken in the northern hemisphere as the polar region warms more than the rest of the hemisphere.

• Extreme weather

This image shows the conclusions of Knutson and Tuleya (2004)
that maximum intensity reached by tropical storms is likely to undergo
an increase, with a significant increase in the number of highly
destructive category 5 storms.

Storm strength leading to extreme weather is increasing, such as the power dissipation index of hurricane intensity.

Kerry Emanuel says that hurricane power dissipation is highly correlated with temperature, reflecting global warming.

• Increased evaporation

Over the course of the 20th century, evaporation rates have reduced worldwide; this is thought by many to be explained by global dimming. As the climate grows warmer and the causes of global dimming are reduced, evaporation will increase due to warmer oceans. Because the world is a closed system this will cause heavier rainfall, with more erosion. This erosion, in turn, can in vulnerable tropical areas (especially in Africa) lead to desertification due to deforestation. On the other hand, in some other areas, increased rainfall has lead to growth of forests in dry desert areas.

2. Destabilization of local climates

A study (published in Science) of changes to eastern Siberia's permafrost
suggests that it is gradually disappearing in the southern regions,
leading to the loss of nearly 11% of Siberia's nearly 11,000 lakes
since 1971.

At the same time, western Siberia is at the initial stage where melting permafrost
is creating new lakes, which will eventually start disappearing as in
the east. Furthermore, permafrost melting will eventually cause methane release from melting permafrost peat bogs.

3. Oceans

The
role of the oceans in global warming is a complex one. The oceans serve
as a sink for carbon dioxide, taking up much that would otherwise
remain in the atmosphere, but increased levels of CO₂ have led to ocean acidification.

Furthermore, as the temperature of the oceans increases, they become less able to absorb excess CO₂. Global warming is projected to have a number of effects on the oceans.

4. Rise in Sea Levels

With increasing average global temperature, the water
in the oceans expands in volume, and additional water enters them which
had previously been locked up on land in glaciers, for example, the Greenland and the Antarctic ice sheets.

An increase of 1.5 to 4.5 °C is estimated to lead to an increase of 15 to 95 cm (IPCC 2001). George Monbiot, a British journalist, summarizes his findings as follows: "The IPCC
predicts that sea levels could rise by as much as 59 cm this century.
Hansen's paper argues that the slow melting of ice sheets the panel
expects doesn't fit the data.

The
geological record suggests that ice at the poles does not melt in a
gradual and linear fashion, but flips suddenly from one state to
another. When temperatures increased to 2-3 degrees above today's level
3.5 million years ago, sea levels rose not by 59 centimetres but by 25
metres."

5. Temperature rise

From
1961 to 2003, the global ocean temperature has risen by 0.10°C from the
surface to a depth of 700 m. There is variability both year-to-year and
over longer time scales, with global ocean heat content observations
showing high rates of warming for 1991 to 2003, but some cooling from
2003 to 2007. The temperature of the Antarctic Southern Ocean
rose by 0.17 °C (0.31 °F) between the 1950s and the 1980s, nearly twice
the rate for the world's oceans as a whole. As well as having effects
on ecosystems (e.g. by melting sea ice, affecting algae that grow on
its underside), warming reduces the ocean's ability to absorb CO₂.

6. Acidification

The
world's oceans soak up much of the carbon dioxide produced by living
organisms, either as dissolved gas, or in the skeletons of tiny marine
creatures that fall to the bottom to become chalk or limestone. Oceans
currently absorb about one tonne of CO₂ per person per year. It is estimated that the oceans have absorbed around half of all CO₂ generated by human activities since 1800 (120,000,000,000 tonnes or 120 petagrams of carbon).

7. Abrupt & Irreversible Effects

Partial loss of ice sheets on polar land could imply metres of sea level rise,
major changes in coastlines and inundation of low-lying areas, with
greatest effects in river deltas and low-lying islands. Such changes
are projected to occur over millennial time scales, but more rapid sea
level rise on century time scales cannot be excluded.

Climate change
is likely to lead to some irreversible impacts. There is medium
confidence that approximately 20- 30% of species assessed so far are
likely to be at increased risk of extinction if increases in global
average warming exceed 1.5-2.5°C (relative to 1980-1999).

8. Methane release from hydrates

Methane clathrate, also called methane hydrate, is a form of water ice that contains a large amount of methane within its crystal structure. Extremely large deposits of methane clathrate have been found under sediments on the ocean floors of Earth.

9. Carbon cycle feedbacks

There
have been predictions, and some evidence, that global warming might
cause loss of carbon from terrestrial ecosystems, leading to an increase of atmospheric CO₂ levels. By the end of the twenty-first century, this additional CO₂
varied between 20 and 200 ppm for the two extreme models, the majority
of the models lying between 50 and 100 ppm. The higher CO₂
levels led to an additional climate warming ranging between 0.1° and
1.5 °C. However, there was still a large uncertainty on the magnitude
of these sensitivities. Eight models attributed most of the changes to
the land, while three attributed it to the ocean. The strongest
feedbacks in these cases are due to increased respiration of carbon
from soils throughout the high latitude boreal forests of the Northern Hemisphere.

10. Forest fires

The IPCC Fourth Assessment Report
predicts that many mid-latitude regions, such as Mediterranean Europe,
will experience decreased rainfall and an increased risk of drought,
which in turn would allow forest fires to occur on larger scale, and
more regularly. This releases more stored carbon into the atmosphere
than the carbon cycle can naturally re-absorb, as well as reducing the
overall forest area on the planet, creating a positive feedback loop.

Part
of that feedback loop is more rapid growth of replacement forests and a
northward migration of forests as northern latitudes become more
suitable climates for sustaining forests.

Other major consequences

1. Economic

As recent estimates of the rate of global warming have increased, so have the financial estimates of the damage costs.

Many estimates of aggregate net economic costs of damages from climate change across the globe, the Social Cost of Carbon (SCC), expressed in terms of future net benefits and costs that are discounted to the present, are now available.

Peer-reviewed
estimates of the SCC for 2005 have an average value of US$43 per tonne
of carbon (tC) (i.e., US$12 per tonne of carbon dioxide) but the range
around this mean is large.

2. Effects on agriculture

For some time it was hoped that a positive effect of global warming would be increased agricultural yields, because of the role of carbon dioxide in photosynthesis, especially in preventing photorespiration, which is responsible for significant destruction of several crops. In Iceland, rising temperatures have made possible the widespread sowing of barley, which was untenable twenty years ago.

3. Transport

Roads, airport runways, railway lines and pipelines, (including oil pipelines, sewers, water mains
etc) may require increased maintenance and renewal as they become
subject to greater temperature variation. Regions already adversely
affected include areas of permafrost, which are subject to high levels of subsidence, resulting in buckling roads, sunken foundations, and severely cracked runways.

4. Floods

For historical reasons to do with trade, many of the world's largest and most prosperous cities are on the coast, and the cost of building better coastal defenses
(due to the rising sea level) is likely to be considerable. Some
countries will be more affected than others ? low-lying countries such
as Bangladesh and the Netherlands would be worst hit by any sea level rise, in terms of floods or the cost of preventing them.

5. Migration

Some Pacific Ocean island nations, such as Tuvalu,
are concerned about the possibility of an eventual evacuation, as flood
defense may become economically unviable for them. In the 1990s a
variety of estimates placed the number of environmental refugees at
around 25 million. (Environmental refugees are not included in the official definition of refugees,
which only includes migrants fleeing persecution.) The
Intergovernmental Panel on Climate Change (IPCC), which advises the
world's governments under the auspices of the UN,
estimated that 150 million environmental refugees will exist in the
year 2050, due mainly to the effects of coastal flooding, shoreline
erosion and agricultural disruption (150 million means 1.5% of 2050's
predicted 10 billion world population).

6. Development

The combined effects of global warming may impact particularly harshly on people and countries without the resources to mitigate those effects. This may slow economic development and poverty reduction, and make it harder to achieve the Millennium Development Goals. In October 2004 the Working Group on Climate Change and Development, a coalition of development and environment NGOs, issued a report Up in Smoke on the effects of climate change on development.

7. Ecosystems

Unchecked global warming could affect most terrestrial ecoregions. Increasing global temperature means that ecosystems will change; some species are being forced out of their habitats (possibly to extinction) because of changing conditions, while others are flourishing. Secondary effects of global warming, such as lessened snow cover, rising sea levels, and weather changes, may influence not only human activities but also the ecosystem. Studying the association between Earth climate and extinctions over the past 520 million years, scientists from University of York write, "The global temperatures
predicted for the coming centuries may trigger a new 'mass extinction
event', where over 50 per cent of animal and plant species would be
wiped out." Plants lag behind, and larger animals' migration is slowed
down by cities and highways.

8. Mountains

Mountains
cover approximately 25 percent of earth's surface and provide a home to
more than one-tenth of global human population. Changes in global
climate pose a number of potential risks to mountain habitats.
Researchers expect that over time, climate change will affect mountain
and lowland ecosystems, the frequency and intensity of forest fires,
the diversity of wildlife, and the distribution of water. Changes in
climate will also affect the depth of the mountains snowpacks and
glaciers. These changes could affect the availability of freshwater for
natural systems and human uses.

9. Ecological productivity

Increasing average temperature and carbon dioxide may have the effect of improving ecosystems' productivity.

This causes the sugars being made to be destroyed, suppressing growth. Higher carbon dioxide concentrations tend to reduce photorespiration.
Satellite data shows that the productivity of the northern hemisphere
has increased since 1982 (although attribution of this increase to a
specific cause is difficult). IPCC models predict that higher CO₂
concentrations would only spur growth of flora up to a point, because
in many regions the limiting factors are water or nutrients, not
temperature or CO₂; after that, greenhouse effects and warming would continue but there would be no compensatory increase in growth.

10. Environmental

Secondary
evidence of global warming ? reduced snow cover, rising sea levels,
weather changes ? provides examples of consequences of global warming
that may influence not only human activities but also ecosystems. Increasing global temperature means that ecosystems may change; some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may flourish. Few of the terrestrial ecoregions
on Earth could expect to be unaffected. Increasing carbon dioxide may
increase ecosystems' productivity to a point. Ecosystems' unpredictable
interactions with other aspects of climate change makes the possible environmental impact of this is unclear, though.

11. Water scarcity

Positive eustacy may contaminate groundwater,
affecting drinking water and agriculture in coastal zones. Increased
evaporation will reduce the effectiveness of reservoirs. The continued
retreat of glaciers will have a number of different impacts. In areas
that are heavily dependent on water runoff from glaciers that melt
during the warmer summer months, a continuation of the current retreat
will eventually deplete the glacial ice and substantially reduce or
eliminate runoff. A reduction in runoff will affect the ability to irrigate
crops and will reduce summer stream flows necessary to keep dams and
reservoirs replenished. This situation is particularly acute for
irrigation in South America, where numerous artificial lakes are filled
almost exclusively by glacial melt.

12. Health

The
most direct effect of climate change on humans might be the impacts of
hotter temperatures themselves. Global warming could mean more cardiovascular diseases, doctors warn. Higher air temperature also increases the concentration of ozone
at ground level. In the lower atmosphere, ozone is a harmful pollutant.
It damages lung tissues and causes problems for people with asthma and
other lung diseases. Rising temperatures have two opposing direct
effects on mortality: higher temperatures in winter reduce deaths from cold; higher temperatures in summer increase heat-related deaths. Palutikof et al.
calculate that in England and Wales for a 1 °C temperature rise the
reduced deaths from cold outweigh the increased deaths from heat,
resulting in a reduction in annual average mortality of 7000, and a
government report shows decreased mortality due to recent warming and
predicts increased mortality due to future warming in the United
Kingdom.

13. Spread of disease

Global warming is expected to extend the favourable zones for vectors conveying infectious disease such as dengue fever and malaria.
In poorer countries, this may simply lead to higher incidence of such
diseases. In richer countries, where such diseases have been eliminated
or kept in check by vaccination, draining swamps and using pesticides, the consequences may be felt more in economic than health terms.