What is the greatest and most significant SCIENTIFIC uncertainty concerning human-caused global climate change?

I would have thought that by far the greatest uncertainty concerns the future implications of climate change and relate to the things that we don’t know, rather than what we do know (or partially know).

The climate is subject to any number of variables that can’t adequately be predicted in advance, it is, after all, a chaotic system. To give you an example, until recently we weren’t aware that global warming would have such a significant impact on Rossby Waves (a type of wind). In turn these waves have increased in intensity and are now regularly opposing the jet stream, this is causing massive weather disruption around the world.

In fact, in recent years the jet stream has been at the centre of more extreme weather events than anything else. It’s a factor in the current US heatwave and was by far the dominant force that led to the Russian heatwave in 2010 and the Pakistani floods that same year.

The human influence on future climates is also of great significance – whether India and China pass Clean Air Acts, if we take steps to reduce greenhouse gas emissions, if we overcome the nuclear fusion containment issue etc.

Another area that isn’t particularly well understood at present concerns the thermohaline circulation. There are opposing theories as to how these currents may be affected as the climate warms up. One argument says that the introduction of cold meltwater from the polar regions will increase the density of the ocean in the affected areas, this will provide greater resistance against the currents which could lead to truncation, deflection or divergence.

Opposing this is the notion that decreased salinity caused by mixing with the fresh meltwater, will decrease the density of the ocean and thus allow for acceleration of the currents at the point of poleward overturning. This would speed up the movement of the currents at one end whilst the feed-in mechanism at the other end would be unchanged. Clearly such a situation couldn’t be maintained – you can’t take more out at one end than you put in at the other. This could lead to the collapse or weakening of the affected currents.

The THC situation is compounded by the fact that the network of currents are interlinked. If one is affected it will have a knock-on effect on the others. At the moment we don’t really have a clue what might happen. Historical evidence shows that the North Atlantic Conveyor has changed in the past and that following this there have been changes to the other ocean currents, but there’s no consistency in the changes and therefore we have little to base future predictions on.

This is an area in which more research is being undertaken but I suspect it will be 20 to 50 years before we know enough about the THC to predict with any accuracy how it may be affected.

Whether or not changes in the THC are significant depends very much on what happens and where it happens. The most likely scenario is that the Gulf Stream is affected (it’s the one that’s been most influenced by past climatic changes) and may well divert in a more easterly direction. This would plunge parts of Europe into another ice-age but at the same time would open up vast tracts of agricultural land in Greenland.

Another major uncertainty concerns something called suprapermafrost taliks. Climate scientists had long predicted that we’d see these occurring but quite what the consequence will be remains unclear.

This type of talik occurs in warming permafrost regions and results in stratification of the ground following the formation of an additional layer beneath the frozen surface which is neither frozen nor thawed (it’s warming which inhibits the usual seasonal refreezing). The heat transferred from the SPT layer may or may not be enough to enhance further melting. If it does then we could see methane released on an unprecedented scale.

Although long predicted, it’s not until recently that SPT have begun to appear in Siberia and Russia. Until now the scientists didn’t have anything in the real world that they could actually study. Research currently being conducted should, in the next few years, provide many answers; but for now this is a grey area.

As to the specific question of which is the greatest uncertainty, I’d say it’s the unknown consequences of climate change. Because it’s such a sensitive system it doesn’t take much to cause disruption, and with each passing year there are new manifestations coming to light that no-one predicted. The worst case scenario is that when the temperature gets to X it acts as a trigger that sets in motion an irreversible sequence of unforeseen events.

I'm going to say the greatest uncertainty in climate science is the effect of aerosols on radiative forcing which has several direct and indirect influences and perhaps a few we are not even aware of yet.

This includes the interactive processes of clouds, aerosols and precipitation and what this means to climate feedback processes and is especially important in determining climate sensitivity. As well, there needs to be a way to separate the size and effect of man made aerosols versus natural ones. http://www.mpg.de/592145/pressRelease20091006

I was going to say cloud feedbacks as the answer but as I thought about it a bit more I think aerosols are a main part of that problem. This figure from the IPCC shows various climate radiative forcings and notice the aerosols and cloud feedback have the largest uncertainty or error bars: http://www.ipcc.ch/publications_and_data/ar4/syr/e...

This is an interesting read and is not too technical: http://www.iac.ethz.ch/edu/courses/master/modules/...

Cloud parameterization has improved dramatically in a decade. Climate sensitivity still carries some large error bars, though mostly on the "more sensitive" side than the "low sensitivity" side, which is tightly bounded (in terms of its localized slope of dT ⁄ dCO₂ over the current levels, anyway.) But without human influence, the need to know it more precisely diminishes, too.

So human behavior in the future is probably the most significant uncertainty, now, I think. This isn't just about atmospheric CO₂, but also about deforestation and habitat modification and a variety of other behaviors that also impact climate. Eliminate human uncertainty and I suspect, despite some non-linearities that we remain ignorant about, more accurate prediction for longer times.

The largest scientific unknown is whether global warming will make internal climate variability larger or smaller. Since models don't current;y do a good job with internal variability in an unforced climate, estimating what happens with increased greenhouse forcing is highly speculative. This is why you see most of the recent research emphasis in the US Global Climate Research Program focus on understanding and modeling internal variability. Pinning down climate sensitivity ties into this as well.

I would say (4) Biological resiliency (or lack thereof).

Climate shifts and the corresponding alterations in distribution of species diversity is very poorly understood. From a discrete standpoint (i.e. the manner in which most studies are executed) the response of biota to perturbations is disconcerting. However, examples of much greater resiliency than expected have been reported over longer (and shorter) units of time.

Considering that the biosphere and atmosphere work in concert, a much greater understanding of the interactions between these two is needed.

I agree with Johnathan. Probably the biggest uncertainty is what we do. It's difficult to predict the future impact of global warming when we don't know what our future response will be.

can the Earth possibly restabalize, is there time to stop the planet from dying or is it already too late.

The most troubling part of this issue is that so much of the "evidence" supporting the idea that human activity causes climate change has been found to be false or intentionally misrepresented. Climate change is a fact. That is why we have had Ice Ages. During the Jurassic period the amount of CO2 in the atmosphere was much higher than it is now. Humans did not cause either of these events. But falsifying evidence to try to prove one opinion or the other poisons the process.

al gore.