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Climate change what we do and dont know Gerard Wynn

Posted on: Tue January 31, 2012

LONDON: The public profile of climate change has slipped as a global economic downturn presses, but that could usefully spur new efforts to narrow the uncertainty that remains over the risks posed by global warming.

The need for clarity has grown for renewable energy in the wake of new fossil fuel and especially shale gas finds which threaten to undermine the energy security case for low-carbon alternatives.

Manmade greenhouse gas emissions are probably responsible for a definite global warming in the past three decades, a tinkering with the world\'s climate which seems dangerous and demands a response.

But even that basic assertion is qualified, and that\'s just the start: climate science deals in uncertainties which include the amount of future warming, regional impacts and so-called tipping points.

Complicating the message, not all the world is warming, and where it is, this may have a beneficial effect in the short term especially in cooler areas.

That leaves space for sceptics. Some 16 scientists doubted any urgency writing in the Wall Street Journal last week, on the basis that the Earth hadn\'t warmed relentlessly since 1998, a very hot year in the long-term record.

Following is a review of the latest science, of what we do and don\'t know, drawn from visits to Britain\'s two main research centres and conversations with scientists and sceptics.

THE WORLD IS WARMING

WE KNOW: Data from about 36,000 weather stations show that the planet has on average warmed in the last century.

The world\'s three main climate research groups have used samples of 3,000-5,000 stations with longer data records to show independently that land and sea surface temperatures have risen by a global average of 0.8 degrees Celsius since 1900 (plus or minus 0.2 degrees).

Temperatures rose from 1910 until the early-1940s, and then after a lull, resumed upwards. Since 1979 temperatures have risen by an average 0.16-0.17 degrees per decade.

Some sceptics had argued the readings were biased by a "urban heat island" effect where growing towns and cities had gradually encroached some weather stations. Cities absorb more heat than trees and fields in the open countryside.

But a University of California research group last year found the effect was unimportant, with identical warming at a sample of rural stations.

The Berkeley group used more advanced statistics to use the full record of 36,000 weather stations, and narrow the error range to 0.9 degrees Celsius average land (not sea) surface warming in the last five decades (plus or minus 0.04 degrees).

WE DON\'T KNOW: How climate change will vary from year to year is unclear because of natural influences on the climate. A linear warming trend is unlikely.

Britain\'s Nigel Lawson, a former energy and finance minister and now vocal climate sceptic, says that there has been "no warming this century". That\'s based on the World Meteorological Organisation observation that the years 1998, 2005 and 2010 were tied for the hottest in a temperature record dating back to about 1850.

However nine out of the 10 hottest years in the temperature record have been in the 21st century - only 1998 was not. 2011 was the ninth hottest so far, according to NASA.

Scientists argue that the world\'s temperature is pulled one way and then the other by natural events, like a large volcanic eruption (cooling) in the early 1990s, and a record El Nino Pacific Ocean weather pattern (warming) in 1998.

CARBON DIOXIDE LEVELS ARE RISING DUE TO MANMADE EMISSIONS

WE KNOW: Levels of carbon dioxide (CO2) in the atmosphere have risen by about two fifths, to 393 parts per million, since pre-industrial times, or by a little over 2 ppm per year.

We also know that this is due to rising human CO2 emissions, up about 3 percent annually, and not from natural systems such as volcanoes. The strongest proof is that far more CO2 is being emitted by human activity than actually accumulating in the atmosphere, with about half going back into the oceans, plants and soil.

CO2 AND OTHER GREENHOUSE GASES ARE RESPONSIBLE FOR WARMING

WE KNOW: Greenhouse gases (GHGs) trap infra-red radiation. Scientists however debate how much we\'re to blame for the observed warming.

Higher levels of carbon dioxide (CO2) and of other GHGs will block heat from leaking into space, and so warm the Earth. Evidence for that blocking is an observed cooling of the upper atmosphere (stratosphere) at the same time as a warming lower down.

A U.N. panel of climate scientists, the Intergovernmental Panel on Climate Change (IPCC), published a consensus view in 2007 that manmade GHGs were "very likely" responsible for most warming in the past five decades, and calculated how much extra energy GHGs added to the Earth, called their radiative forcing.

Scientists say their climate models can only explain the most recent warming if they include that greenhouse gas effect.

Scientists can work out what GHG emissions mean for global temperature rises, called the "climate sensitivity" for every doubling of atmospheric CO2. This can be calculated by observing how much CO2 and other forces have changed the Earth\'s temperature in the distant past.

Scientists calculate, and some sceptics agree, that the climate sensitivity of CO2 is about 1.2 degree Celsius, before feedbacks and wider effects are included.

WE DON\'T KNOW: Still unclear is exactly how much GHGs will warm the planet because of these other, poorly understood effects.

Firstly, GHGs have to be disentangled from manmade and natural effects. For example, manmade smoke is probably cooling the planet (see below), an effect which has to be added to calculate the warming effect of GHGs.

Secondly, global warming will itself trigger yet more warming (positive feedbacks). Those feedbacks also have to be added to calculate the impact of GHG emissions. There may also be negative feedbacks, which will slow climate change.

Most scientists agree that the total climate sensitivity of CO2 is about 2-4 degrees Celsius warming, and perhaps even more over the longer term because some effects take decades to kick in. In other words, positive feedbacks make GHGs much more of a problem.

CLIMATE CHANGE FEEDBACKS

WE KNOW: Climate change can itself amplify warming through so-called positive feedbacks which make human GHGs more dangerous.

Climate scientists calculate that the main feedbacks double the climate sensitivity of CO2 to about 2.4 degrees Celsius.

These are, first, a well-understood increase in water vapour, a powerful GHG, as a result of more evaporation as the Earth warms. Basic physics says that atmospheric water vapour increases by about 6 percent per degree of warming.

The second feedback is annual summer melt of Arctic sea ice: global warming is expected to speed up that melt, leaving more open water which in turn will reflect less sunlight back into space, leading to more warming.

WE DON\'T KNOW: the definite climate sensitivity of CO2.

Climate scientists say the uncertainty is mostly about positive feedbacks, meaning the risk is on the upside for more warming rather than less.

Those uncertain feedbacks include:

1. Cloud cover: scientists expect global warming to change cloud formation, but don\'t know exactly how or where.

More clouds at lower atmospheric levels would reflect more sunlight back into space, cooling the Earth, while more clouds at higher levels would trap heat, warming it up.

This uncertainty is the main cause for the broad range of projections for warming later this century. The latest thinking is that cloud cover is a weakly positive feedback, according to Britain\'s Met Office Hadley Centre.

2. The Earth\'s natural carbon cycle: at present, oceans, soil and plants absorb about half of all human carbon emissions.

Over time, these systems may become saturated, but it\'s difficult to be sure if or when this will speed up climate change.

For example, regarding plants, there\'s a balance between a short-term increase in CO2 absorption, as plants benefit from more CO2 in the air (a negative feedback), and in the medium term higher temperatures which may damage growth and lead to CO2 emissions as plants die (positive feedback).

3. Global warming may lead to the rapid release of the powerful GHG methane, for example in the Arctic as permafrost "caps" over the methane melt, but no clinching evidence has emerged.

Scientists have reported the release of methane from the Arctic Ocean seabed off eastern Siberia, but the trend and cause are unsure.

4. To use a phrase favoured by former U.S. defence secretary Donald Rumsfeld, there are "unknown unknowns". We simply don\'t what other positive or negative feedbacks there might be, as the world warms further, either making climate change worse or slowing it down.

HUMAN-CAUSED COOLING EFFECTS

WE KNOW: People can actually cool the Earth, as well as produce warming greenhouse gases.

Unfiltered coal plants spew sulphur and smoke (called aerosols) into the air. These reflect sunlight back into space, and also generate water droplets in the air which do the same.

WE DON\'T KNOW: It remains unclear exactly how much aerosols cool the Earth. Scientists say that aerosols help explain a planetary cooling in the 1940s, and a subsequent lull in warming, after the United States and Europe built a new generation of coal-fired power plants and before they introduced clean air laws in the 1970s.

Some scientists also blame a more recent lull in observed warming partly on a recent build-out of Chinese coal plants.

Sceptics have suggested that such arguments are desperate attempts by scientists to make their models fit the data.

NATURAL INFLUENCES ON CLIMATE

WE KNOW: The sun can\'t explain recent global warming because changes in solar output have not matched the temperature trend.

That appears also to rule out a role for cosmic rays from outer space, which can impact cloud formation, but whose influence is tied to the sun which in its active phase deflects the rays.

It seems the oceans aren\'t responsible either: water to a depth of 700 metres has warmed alongside surface temperatures, ruling out some kind of natural cycle where oceans had spewed heat into the atmosphere.

Meanwhile, some scientists say ocean currents historically haven\'t had a strong enough temperature impact to account for the recent warming.

Natural influences do change the Earth\'s climate in the short-term. These include the Pacific El Nino weather pattern (warming); the opposite La Nina (cooling); solar cycles (warming during a solar maximum) and major volcanic eruptions (cooling).

WE DON\'T KNOW: exactly how far these natural effects change the climate.

IS THE PRESENT WARMING TREND UNUSUAL?

WE KNOW: It is almost certain that temperatures were much higher in the distant past, 50 million years ago and more, according to various indirect temperature data.

WE DON\'T KNOW: It is unclear whether temperatures now are higher than a more recent so-called "Medieval Warm Period" (in some parts of the world about 1,000 years ago). Some evidence including tree rings, ice cores suggest that today is warmer.

Sceptics say that warming now may be just another kind of Medieval Warm Period. They argue that if we don\'t understand what happened then, we can\'t rule out the same thing now.

Similarly, temperatures in parts of the world dipped at times around 1600-1700, and sceptics have suggested that present warming may somehow mark a continuing recovery from that cold period, coined the "Little Ice Age".

Scientists say that regardless of temperature changes a few centuries ago, there is no evidence of a relationship with more recent warming, which instead is explained by forcing from greenhouse gas emissions.

WILL CLIMATE CHANGE BE DANGEROUS THIS CENTURY?

WE KNOW: Rising emissions should lead to more global warming, with the present trend nearly 0.2 degrees per decade.

In terms of impacts, already there is a trend of more frequent, extremely hot heatwaves such as that in Moscow in 2010, which have led to loss of life and crops.

Changes in rainfall patterns have also been observed, such as more daily variation in the Indian monsoon. Such volatility may affect crop yields because most plants prefer a steady flow of water rather than dry spells in the middle of a growing season sprinkled with floods.

More droughts will lead to less available water for irrigation and so on, especially in areas that are already rather dry.

Climate change will have benefits too. A British report published last week said it could increase crop yields and cause fewer winter deaths, alongside negative impacts and in particular damage from more severe floods.

WE DON\'T KNOW: More work is needed on specific impacts, especially the narrower the regional focus, and the further out in time.

The British impact study summed it up nicely: "There is a greater likelihood that summers will be drier, but projections cover a range of outcomes including wetter summers."

It\'s impossible to be sure whether any individual weather event was due to climate change, although some peer-reviewed papers have calculated the probability of that, for example for the Moscow heatwave (80 percent probability due to climate change, according to one study), as well as the European 2003 heatwave, and individual British floods.

Sea level is already dangerous to some communities living in low-lying islands.

But bigger sea level rise is some way off: some scientists warn that the Greenland ice sheet will melt completely over many centuries or millennia adding 7 metres to sea levels, if long-term warming exceeds 2 degrees Celsius or so, in one of the most difficult impacts to prepare for but also the furthest off.

Another ocean risk is acidification, where the sea gradually absorbs more and more CO2, becoming more acidic, which could damage corals and shellfish and upset marine food chains and fish stocks.

Courtesy : Samaa News