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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 allows a 3.5oC rise! Some respected scientists believe there is a real chance we will render the Earth not just unpleasant compared with now, not just uninhabitable by humans, but completely lifeless.
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,
- because 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
- because 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!
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.
Useful links
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
Climate simulation using C-ROADS
Some useful links debunking various arguments used by climate change deniers:
http://www.skepticalscience.com/argument.php
http://www-personal.buseco.monash.edu.au/~BParris/BPClimateChangeQ&As.html
http://www.newscientist.com/article/dn18279
A thorough demolition of denialist Ian Plimer's novel "Heaven and Earth": http://www.complex.org.au/tiki-download_file.php?fileId=91
More links can be found here:
Sutherland CAN
Bush's advisors as lobbyists for the carbon mafia
Popular denialists' arguments
"Global warming has plateaued"
"Climategate"
"Glaciergate"
"CO2 increase is effect, not cause"
"Missing 'hotspot' fingerprint"
"Australia's emissions are insignificant"
"Greenhouse effect of CO2 already saturated"
"Arctic sea ice has stabilised"
"Upper Troposphere is Drying"
"IPCC overestimates CO2 growth"
Climate Change deniers' websites:
A Cool Look at Global Warming
JoNova's Skeptics Handbook
Lord Monckton & SPPI
Miscellaneous facts and fallacies
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 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.
