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Do we need to reduce GHG emissions?
There are many parts to this question:
Is global warming happening?
To get a complete measure of the heat in the biosphere you would need temperatures at billions of locations scattered through the atmosphere, land and oceans. You'd also need a good estimate of the total ice at the poles and in glaciers. (If you heat ice that's at 0oC it does not warm up, it melts. Only when it has all melted does the temperature start going up again.)
But there are several ways to spot trends. As well as direct measurents, we can look at ocean levels and changes in plant distributions and animal behaviours. Naturalists observe a shift in the seasons. Ocean levels are rising, which means either they're warming up or land ice is melting and flowing into them, or both.
There can be no doubt the world has warmed in the past 100 years. Some deniers claim it stopped warming around 10 years ago, but that claim is shot down quite easily. Anyone who persists with this argument must be immune to logic.
Can such warming be caused by GHGs (greenhouse gases)?
In 1896 Svante Arrhenius was thinking about ice ages. He calculated that halving the CO2 in the atmosphere would cool Europe some 4-5°C. Since then, fluctuations in CO2 levels over millions of years have been linked to temperature changes. (Details)
Can anthropogenic GHGs warm the planet significantly?
Arvid Högbom then calculated that if we were to burn all the fossil fuel available to us it would produce 10 times the CO2 already in the atmosphere. This definitely had the potential to change the climate, but at the rate at the time it would have taken millennia to burn it all. At the current rate of growth it will take a few hundred years, and climate may be affected much sooner.
It is vital to understand that the rate of emissions only affects when it will become a problem, not if. Whether water gushes into a bath or only drips, it will fill it eventually. For this reason, every country needs to aspire to being carbon neutral.
Have they warmed it significantly over the last century?
Note that the question is not whether human emissions are the main cause, only whether they have made a significant contribution. Whatever the main cause, if we can mitigate it by reducing emissions then that's what we should do.
Over the years, a vast amount of work has gone into developing models of the climate. These get better all the time. They are assessed by their abilities to "predict" past changes, as known from the geological record, from what is known of the circumstances of those times: atmospheric mix, arrangements of the continents, strength of the sunshine and so forth. None of them are perfect, but increasingly they agree. And most agree that our emissions have been enough to account for the increased GHGs in the atmosphere, and that these in turn are sufficient to have driven the increase.
How bad will it get if we carry on as now?
With business as usual, the models predict the temperature will rise several degrees over the next 100 years or so. They indicate that even a 2oC rise is risky. The consensus on the recent Copenhagen accord is that it allows a 3.5oC rise! Predicting the consequences is much harder. Even small long-term changes in temperature can shift winds and ocean currents, leading to quite different patterns of climate. Warmer polar waters could cause a permanent El Niño.
What is a safe level in the atmosphere?
At first, the IPCC thought 450-550 ppm was ok, but revised it down to 450ppm. That number seems to have become etched into the minds of many politicians, but meanwhile the scientists have had cause to bring it down to 350ppm. Several propose 300ppm as the completely safe upper limit. The pre-industrial level was about 280, and right now we're approaching 390. If we're already past the safe limit, why aren't we seeing major disasters right now? Because there's a lot of lag in the system. If you increase the level of GHGs in the atmosphere quickly then keep it the same for a while the Earth gets slowly hotter for decades. We've left matters so late that now we don't only need to become carbon neutral but must actually draw down some.
Unfortunately, we're still not sure how sensitive the temperature is to the GHG level. Existing models say the Earth should have warmed 2oC since pre-industrial times, but it has only warmed 0.8oC. Is this because
- the GHGs are not as powerful as we think, in which case maybe 450ppm is ok,
- the heat is going somewhere we can't observe so easily (more ocean mixing, deep ice warming...), in which case there's more lag in the system, but we still need to act strongly now, or
- pollution haze is blocking the sun, so temperatures will climb rapidly as the air gets cleaner?
Of these, the biggest uncertainties in the science are regarding pollution haze, so there's no cause for complacency.
Remember 350 ppm!
Feedbacks
Various processes both affect the earth's temperature and are affected by it. So when the temperature changes, the chain of events can lead to another change later on. If the second change is in the same direction as the original one, amplifying it, it is call a positive feedback, otherwise it is a negative feedback. Note that a negative feedback cannot result in the temperature going down instead of up; it only reduces the amount of change.
What matters about a feedback is
- Is it positive or negative, and how strong is it?
This can be complicated because a feedback can vary in strength, and even direction, according to circumstances
Here are some known feedbacks:
Negative, very strong, immediate. Though perhaps not usually thought of as a feedback, this is the basic mechanism that keeps the earth at a more-or-less constant temperature day-to-day. The hotter the earth, the more heat is radiated into space.
Positive, strong, short term. When the temperature rises, more water evaporates into the atmosphere. It may feel drier because the air is less saturated, but in absolute terms there is more water vapour, and this is a very powerful greenhouse gas.
Strong, positive in the medium term. When the temperature rises, CO2 in the atmosphere increases over the following hundreds of years. This is the result of several separate processes, some of which may in themselves be negative.
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-/+ Plant growth is stronger in temperate zones when warmer, taking up CO2. But too much warming can make it a positive feedback.
But much longer term there is a strong negative feedback. A hotter earth has more active weather systems, eroding rocks. The exposed rock absorbs CO2, sequestering it. This process takes millions of years, but is thought to be a vital thermostat keeping the earth habitable over billions of years, even though the sun has gradually warmed to be 30% hotter today.
http://www.sciencedaily.com/releases/2010/06/100617143936.htm
Positive, strong, short term. A cover of sea ice reflects the sun's rays back into space, keeping the planet cool.
Positive, unknown strength, various timescales. Clathrates are frozen mixtures of methane and water which can lie at the bottom of oceans. Permafrost also retains methane. Warming will melt more of these, releasing the methane into the atmosphere. How much is there and how quickly they will melt is uncertain.
There are signs that more mixing is occurring between arctic air and the northern temperate zone. While this produces cold weather in Europe and North America, it warms the arctic, leading to global warming as described above. If this mixing is caused by the warming then it is another positive feedback path.
Ocean Acidification
Even if global warming were not an issue, our CO2 emissions are seriously damaging ocean life. Denialists don't usually discuss this because there are far fewer opportunities for sowing doubt. CO2 increases the acidity, making it harder for corals and shellfish to grow, and even dissolving them.
http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=12904
http://www.sciencedaily.com/releases/2010/04/100420152841.htm
What is a sceptic?
Climate Change deniers prefer to be called sceptics. A sceptic, though, is someone who believes something only when the evidence for it outweighs the evidence against. All scientists worthy of the term are sceptics by definition, and the majority are persuaded of anthropogenic (human-induced) global warming. For most deniers the scepticism is entirely one-sided, not being sceptical for a moment about their own position.
Certainly there are some serious scientists who doubt AGW, and these must be given due consideration. Meanwhile, major policy decisions cannot wait for unanimous agreement. Politicians must judge the consensus. This analysis should help: http://www.pnas.org/content/early/2010/06/04/1003187107.abstract.
Useful links
19th May, 2010 : Three reports from the US National Research Council
Advancing the Science of Climate Change
Limiting the Magnitude of Future Climate Change
Adapting to the Impacts of Climate Change
A history of the science http://www.aip.org/history/climate/co2.htm
Good recent updates http://www.copenhagendiagnosis.org/, and http://www.parliament.vic.gov.au/research/2009CIClimateChange.pdf
The scientists write: http://www.ucsusa.org/global_warming
Climate simulation using C-ROADS
Earth - The Power of the Planet (BBC DVD, 2007)
Some links debunking various arguments used by climate change deniers:
Dave Rado on Martin Durkin's film, the Great Global Warming Swindle.
Howard Friel on Bjørn Lomborg.
Ian Enting on Ian Plimer.
http://www.cana.net.au/public-access/busting-myths-climate-deniers
http://www.cana.net.au/sites/default/files/DoubtingAustralia.pdf
http://www.greenpeace.org/raw/content/international/press/reports/dealing-in-doubt.pdf
http://www.guardian.co.uk/environment/blog/2010/feb/17/iphone-app-climate-change
http://www.skepticalscience.com/argument.php
http://scienceblogs.com/illconsidered/
http://www-personal.buseco.monash.edu.au/~BParris/BPClimateChangeQ&As.html
http://www.newscientist.com/article/dn18279
A detailed deconstruction of Viscount Monckton: http://www.stthomas.edu/engineering/jpabraham/
A thorough demolition of denialist Ian Plimer's novel "Heaven and Earth": http://bravenewclimate.com/2009/04/23/ian-plimer-heaven-and-earth/
A critique of Leon Ashby's "Why an Emissions Trading Scheme (ETS) is not necessary":
http://sites.google.com/site/sutherlandcan/sceptics/ashby-response
More links can be found here:
Sutherland CAN
Bush's advisors as lobbyists for the carbon mafia
Greenpeace's exposure of denialist funding
Popular denialists' arguments link
Climate Change deniers' websites:
A Cool Look at Global Warming
JoNova's Skeptics Handbook
Lord Monckton & SPPI
Climate Sceptic Show
Miscellaneous facts and fallacies
1. Will GW shut down the Gulf Stream and cause an Ice Age in Europe?
Yes, it could shut down the Gulf Stream, and yes, that might make temperatures a few degrees cooler in Western Europe than GW would otherwise have made them. But stopping the Gulf Stream will not cause an Ice Age. Most of the winter temperature difference between Eastern seaboard US and Western seaboard Europe is caused by the prevailing winds being West to East. The Atlantic ocean acts as a huge heat buffer, tending to smooth out the year-round temperatures, and the Westerly winds transport this smoothing to Europe. The Gulf Stream helps, but is not the major component.
http://www.americanscientist.org/issues/pub/2006/4/the-source-of-europes-mild-climate/1
That said, GW could change the wind patterns in ways that are hard to predict.
2. Is GW responsible for increased earthquakes and volcanoes?
First, there have not been more earthquakes and volcanoes than normal in recent years. Secondly, it would be very hard to relate any given such event to global warming.
That said, there are reasons to expect that GW will lead to increased seismic activity. Melting of thick ice cover could release magma; conversely, rising sea levels could mean fewer eruptions from coastal volcanoes; changing wind patterns alter the ocean's pressure footprint on the crust, triggering earthquakes and tsunamis.
