Climate Change

Introduction:

Evidences that the climate is changing

In 1988, the United Nations Environment Programme and the World Meteorological Organization set up the Intergovernmental Panel on Climate Change (IPCC) to examine the most current scientific information on global warming and climate change. More than 1,250 authors and 2,500 scientific experts’ reviewers from more than 130 countries contributed to the panel’s most recent report, Climate Change 2007: The Fourth Assessment Report (the full report will be released in November 2007). The IPCC Fourth Assessment Report is the most comprehensive and up-to-date evaluation of global warming. As the new benchmark, it serves as the basis for international climate negotiations.

The IPCC concluded in its Third Assessment Report, “An increasing body of observations gives a collective picture of a warming world and other changes in the climate system.” The kinds of changes already observed that create this consistent picture include the following:
Examples of observed climatic changes
- Increase in global average surface temperature of about 0.6 °C in the 20th century
- Decrease of snow cover and sea ice extent and the retreat of mountain glaciers in the latter half of the 20th century
- Rise in global average sea level and the increase in ocean water temperatures
- Likely increase in average precipitation over the middle and high latitudes of the Northern Hemisphere, and over tropical land areas
- Increase in the frequency of extreme precipitation events in some regions of the world
Examples of observed physical and ecological changes
- Thawing of permafrost
- Lengthening of the growing season in middle and high latitudes
- Pole ward and encroachment of plant and animal ranges
- Earlier flowering of trees
- Earlier emergence of insects
- Earlier egg-laying in birds

What makes the climate change?

The Earth’s climate is influenced by many factors, mainly by the amount of energy coming from the sun, but also by factors such as the amount of greenhouse gases and aerosols in the atmosphere, and the properties of the Earth’s surface, which determine how much of this solar energy is retained or reflected back to space.

The atmospheric concentrations of greenhouse gases such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) have significantly increased since the beginning of the industrial revolution. This is mainly due to human activities, such as the burning of fossil fuels, land use change, and agriculture. For instance, the atmospheric concentration of carbon dioxide is now far higher than in the last 650 000 years and has been growing faster in the last ten years than it has been since the beginning of continuous measurements around 1960.

It is very likely that, overall, human activities since 1750 have had a global warming effect on the Earth.

What changes have been observed so far in climate? What is causing the present-day changes in climate?

Observed changes in climate

The warming of global climate is now unequivocal. There are many observations of increasing air and ocean temperatures, widespread melting of snow and ice, and rising sea levels.

More specifically, eleven of the last twelve years (1995-2006) rank among the 12 warmest years ever recorded since global surface temperatures are measured (1850). Over the last 100 years (1906–2005), global temperature has increased by 0.74°C. Global sea level has risen by 17 cm during the 20^th century, in part because of the melting of snow and ice from many mountains and in the polar regions. More regional changes have also been observed, including changes in Arctic temperatures and ice, ocean salinity, wind patterns, droughts, precipitations, frequency of heat waves and intensity of tropical cyclones.

The temperatures of the last half century are unusual in comparison with those of at least the previous 1300 years. The last time that the polar regions remained significantly warmer than now for a very extended period (125 000 years ago), the sea level rose by 4 to 6 meters.

What are the projected changes for the 21st century and on the longer term?

Projected changes in temperature for the 21^st century

The global average temperature is expected to increase by about 0.2°C per decade over the next two decades. Continuing greenhouse gas emissions at or above current rates would cause a further increase in global temperatures and many other climatic changes during the 21^st century.

The best estimates for projected global temperature increases from the 1980s to the end of the 21^st century range from 1.8°C (1.1 – 2.9°C) to 4°C (2.4 – 6.4°C) for the IPCC scenarios that do not consider additional mitigation measures apart from those already in place in 2000.

Other projected changes for the 21^st century

Global average sea level is expected to rise by 18 to 59 cm by the end of the 21^st century. Warming is expected to be greatest over land and at high northern latitudes and smallest over the Southern Ocean and parts of the North Atlantic Ocean. Other projected changes include acidification of the oceans, reduced snow cover and sea ice, more frequent heat waves and heavy precipitation, more intense tropical cyclones, and slower oceanic currents.

Projected changes on the longer term

Warming and sea level rise caused by human activities will continue for centuries, even if greenhouse gas concentrations were to be stabilized. If warming persists over many centuries, it could lead to a complete melting of the Greenland Ice sheet, increasing global sea levels by about 7m.

What factors determine Earth’s climate

The climate system is a complex, interactive system consisting of the atmosphere, land surface, snow and ice, oceans and other bodies of water, and living things. The atmospheric component of the climate system most obviously characterizes climate; climate is often defined as ‘average weather’.

Climate is usually described in terms of the mean and variability of temperature, precipitation and wind over a period of time, ranging from months to millions of years (the classical period is 30 years).

The climate system evolves in time under the influence of its own internal dynamics and due to changes in external factors that affect climate (called ‘forcings’). External forcings include natural phenomena such as volcanic eruptions and solar variations, as well as human-induced changes in atmospheric composition. Solar radiation powers the climate system.

There are three fundamental ways to change the radiation balance of the Earth:

1. by changing the incoming solar radiation (e.g., by changes in Earth’s orbit or in the Sun itself);
2. by changing the fraction of solar radiation that is reflected (called ‘albedo’; e.g., by changes in cloud cover, atmospheric particles or vegetation);
3. by altering the long wave radiation from Earth back towards space (e.g., by changing greenhouse gas concentrations).

Climate, in turn, responds directly to such changes, as well as indirectly, through a variety of feedback mechanisms.

1. What is the greenhouse effect?

The Sun’s warmth heats the surface of the Earth, which in turn radiates energy back to space. Some of this radiation, which is nearly all in the infrared spectrum, is trapped in the atmosphere by greenhouse gases. For instance, water vapor strongly absorbs radiation with wavelengths between 4 and 7 micrometres, and carbon dioxide (CO₂) absorbs radiation with wavelengths between 13 and 19 micrometres.

The trapped radiation warms the lower atmosphere, or troposphere. Some heat then finds its way back down to the Earth’s surface, making it hotter than it would otherwise be. This is the greenhouse effect.

1. Are water vapor and carbon dioxide all we have to worry about?

No. Other gases can absorb infrared radiation and contribute to greenhouse warming. These include methane, ozone, CFCs (chlorofluorocarbons) and nitrous oxide (released by fertilisers). Methane is the most important of these. Its atmospheric concentration has more than doubled since pre-industrial times. Methane sources include bacteria in paddy fields, cattle guts and natural gas from landfills and rotting vegetation. Molecule for molecule, other substances are even more potent greenhouse gases. A single molecule of either of the two most common CFCs has the same greenhouse warming effect as 10,000 CO₂ molecules.

1. Is the greenhouse effect a thoroughly bad thing?

Not quite. Without it, the planet would not be warm enough to support life as we know it. The problem is that pre-industrial greenhouse gas levels are being boosted by burning fossil fuels. If nothing is done to curb emissions, the amount of CO₂ in the atmosphere will probably be more than double pre-industrial levels by the end of this century.

1. How do we know what pre-industrial greenhouse gas levels were?

The most informative measurements have come from air bubbles trapped in Antarctic ice. These show that, for at least 400,000 years, CO₂ levels in the atmosphere have closely followed the global temperatures as recorded in ice cores, tree rings and elsewhere.

5. If measuring greenhouse gas levels is so precise, why is there so much confusion and uncertainty over global warming?

There is no easy formula for predicting what CO₂ increases will do to global temperatures. While we can calculate that a doubling of atmospheric CO₂ will force roughly 1°C of warming, the planet is more complex than that. It could magnify the effect, but it could also conceivably dampen down warming. Global processes such as the formation of ice and clouds, the circulation of the oceans and biological activity all interact to provide feedback effects.

6. What effects are global warming feedbacks likely to have?

One of the easiest to estimate is the “ice-albedo” feedback. As the world warms, ice caps will melt, to be replaced by water or land. Ice is very efficient at reflecting solar radiation, whereas water and land are less so. Therefore, the Earth’s surface will trap more heat, increasing warming – a positive feedback.

Less clear-cut is the impact of the extra water vapor likely to enter the atmosphere because of higher evaporation rates. This added water vapor itself contributes to the greenhouse effect, another positive feedback. But it may also increase cloud cover, shrouding and cooling the Earth – a negative feedback.

Disputes about how water vapor and clouds will influence global warming are at the heart of disputes between mainstream scientists and the handful of greenhouse sceptics. Most believe that positive feedbacks could amplify the warming effect by between two and five times. But some skeptics believe the feedback effect could be neutral or negative.

7. Are there scientists out there who do not believe in the greenhouse effect or global warming?

No, this is a myth. All scientists believe in the greenhouse effect. Without it the planet would be frozen. And all scientists accept that if humans put more greenhouse gases into the atmosphere then it will warm the planet. The only disagreement is over precisely how much the warming will be amplified by planetary feedbacks.

However, there is a growing consensus that the average global warming of 0.6°C seen in the twentieth century – and particularly the pronounced warming of the past three decades – is due to the greenhouse effect.

8. Are there other greenhouse gas complications?

Yes. A whole series of other feedbacks will influence the concentration of greenhouse gases in the atmosphere. Not all the CO₂ that we put into the atmosphere stays there. Some is absorbed by vegetation and a lot is taken up by the oceans. If CO₂ absorption rate changes, then the rate of build-up in the atmosphere will also change, potentially speeding up, or slowing down, global warming.

One way to increase the build-up of CO₂ would be to chop down all the tropical forests. Another could be the impact of warming on ocean currents, particularly the global “conveyor belt” that begins in the North Atlantic. This water carries dissolved CO₂ with it on a centuries-long journey across the ocean floor.

Most oceanographers believe that as warming takes hold, and ice formation is reduced, these currents – which lock CO₂ up in the depths – could slow down or carry less water, meaning that less CO₂ is removed from the atmosphere.

9. Is there any evidence of a speed-up in the accumulation of CO₂ in the atmosphere?

Yes. Since the start of the 21st century, the rate of accumulation has accelerated. It is now at twice the 1990s level. Nobody is sure why. It is not because emissions have accelerated. It could be temporary natural variability. Or it could be that the forests and oceans are losing the ability to absorb our pollution. If so, then global warming could shortly gather pace.

10. This is all very pessimistic. Is it not true that a warmer planet will absorb more carbon dioxide?

That is correct. Warmer temperatures and the fertilizing effect of CO₂ in the air will stimulate faster plant growth, which in turn will soak up some of the CO₂. But plants need other things too. They need water, which could be in short supply as greater evaporation will dry out soils, and space, which urbanization is taking up.

11. How do organisms in the oceans affect global warming?

Once dissolved in surface waters, a great amount of CO₂ is absorbed by plankton and other marine organisms and turned into organic compounds. Most of this eventually falls to the ocean floor. The strength of this sink for carbon depends on how much life the ocean is producing. It is not clear to what extent global warming will affect the oceans’ biological productivity – it could rise or fall.

12. Is there anything else that could shield us from global warming?

Yes, volcanoes. When Mount Pinatubo erupted in 1991, it threw masses of sulphate particles and dust into the stratosphere that partially shielded the Earth from solar energy. Computer models successfully predicted that the debris would temporarily cool the Earth’s atmosphere. The models also predicted that as the volcanic debris cleared in 1992 and 1993, average temperatures would swiftly return first to the level of the 1980s, and then, by the mid-1990s, to the higher levels expected with the ongoing build-up of greenhouse gases.

13. Volcanoes produce cooling sulphate particles, but do we make them, too?

Yes we do. Ironically, burning fossil fuels produces sulphate particles. These particles – which make acid rain – help to shield industrialized countries from global warming’s full impact. In some places, such as central Europe and parts of China, they may have even produce a net cooling effect. Dust from soil erosion and desertification can also curb local warming effects.

But even if you are comfortable with the idea of using one form of pollution to protect us from another, there is a problem. Whereas the average CO₂ molecule in the atmosphere lasts for about a century, sulphates and their like persist for only a few days. If you turned down the power stations, the world would get much hotter within a few days. So sulphates are not a solution.

14. How are temperatures predicted to rise over the next few centuries?

This depends on whether we halt the growing concentration of greenhouse gases in the atmosphere. Some warming is inevitable – there are time lags in the natural systems which store up warming for future decades. CO₂ concentrations are currently about 35% above pre-industrial levels, storing up perhaps another degree of warming.

If we can stabilize atmospheric CO₂ concentration by the end of this century, below twice pre-industrial levels, we can probably limit warming to under 5 degrees. But because the gas stays in the atmosphere for a century or more, stabilization requires cutting emissions by 70% to 80%. A tall order. However, some models predict temperature rises of 8 to 10 degrees within 200 years if we do not kick the carbon habit.

15. What are some of the most significant effects global warming will have on the human race?

Unusual droughts are causing serious problems for farming in many regions. Whole countries could get swallowed up by this process, triggering poverty and mass migrations. Super-hurricanes could make other places uninhabitable.

As rainfall patterns alter, rivers will dry up in some regions, while others will flood. Rising sea levels will wipe out many islands and flood low-lying areas, from Bangladesh to the US. As ever, the poor will be most vulnerable. At least in the early decades, rich nations may cope, but ultimately even they could be undermined. Human civilizations have developed over the past 10,000 years – since the end of the last ice age – in an era of generally stable climate. We just do not know how well we will cope with a radical change to the climatic status quo.

16. Will there be global warming everywhere?

Climate modelers admit to uncertainties over how it will affect particular regions. This is because much of our weather depends on circulation patterns, which could alter unexpectedly. Crude estimates suggest that coastal regions may become wetter, while continental interiors become drier, causing deserts to expand. Warming will probably be greatest in Polar Regions, mirroring climate changes already seen this century in both the Arctic and Antarctic.

Local climate could also be altered by changes in ocean circulation. Western Europe is particularly vulnerable. At present, it is kept exceptionally warm in winter by the Gulf Stream, which is part of the ocean conveyor belt mentioned above. Take that away and British weather would be more like Canada’s frigid Hudson Bay, found at the same latitude.

Ice cores reveal growing evidence of sudden shifts in climate over the past 10,000 years that have occurred within a few decades as a result of “flips” in ocean circulation. But most models suggest that the Gulf Stream will not turn off for at least another century.

17. Are there any cataclysmic events in the offing?

One fear is that the entire West Antarctic and Greenland ice sheets might disappear into the oceans. According to projections by the UK’s Hadley Center for Climate Prediction, before the end of the century a warming of at least 3°C would probably trigger the eventual melting of the entire Greenland ice sheet. The glacial collapse would take hundreds of years, but could raise sea levels by 6 meters.