Spartacus made a very reasonable comment, and I agree that I have been trading science for dogma with the faithful. We'll try to limit this thread to those things that are known rather that what may lurk around the next corner, such as the clathrate bomb effect etc. We could also talk about what measures we can take to mitigate or prevent some of these effects.

I'll ask some of the regular contributors to add to this thread, but to improve the signal to noise level, I'll start a sticky thread with the serious responses based on factual evidence.
In terms of the effects of rapidly rising CO2, probably some of the most serious effects are in the oceans.

Certain important marine organisms rely on calcification - the formation of hard structures of calcite or aragonite by extracting the calcium from seawater. Increasing CO2 reduces the ability of these organisms to form these structures. These include certain corals and a large number of pelagic fish, which form "otoliths" which enable the fish to swim upright. With reduced calcification rates, the viability of many of these species is threatened. 

The warming of the oceans will also result in the  differentiation of whole ecosystems. Slow moving benthic organisms that form part of the ecosystem will move much slower than for example fish.  This is likely to impact on the viability of the ecosystems.

From a human perspective, a great many of these organisms are important food species.

I suppose what I was really after was to get some sense of what climate scientists would say or are saying would happen if we delay taking any meaningful climate action  for another 6 years.  By that I don't mean what effects will we see in 6 years time but rather what will we see in the long term (say by 2050) if we do nothing for another 6 years?

ImSpartacus2 wrote on Aug 30^th, 2013 at 9:48am:

I suppose what I was really after was to get some sense of what climate scientists would say or are saying would happen if we delay taking any meaningful climate action  for another 6 years.  By that I don't mean what effects will we see in 6 years time but rather what will we see in the long term (say by 2050) if we do nothing for another 6 years?

None claim to know for certain. Results to date have been pretty alarming. The best I've heard is that, the way we know where a threshold point is, is after we've passed it.

I started this thread with one example.  Here are some broad categories:
http://www.grida.no/graphicslib/thumbs/1805c933-493c-4b85-be16-ad06eb342332/medium/projected-impacts-of-climate-change_154e.jpg

This site provides a reasonably good summary of the risks:
http://climate.nasa.gov/effects

Our sustainability crisis didn’t start and doesn’t stop at climate change

Steb Fisher 9 September 2013, 6.35am AEST

What would happen to the world if, with the snap of our fingers, we shifted all our energy supplies to renewable sources overnight? You might be surprised at the answer: not much, at least for biodiversity and ecosystems.

Certainly, it might solve the climate problem, but I have canvassed this question in a number of different places, and the answers usually converge on this: we would still wreck Earth’s ecosystems. And what’s more, we’d still wreck them on a timescale similar to the trajectory that we’re on already.

The reason is that climate change is a problem, not the problem. At the moment much of the focus is on climate and there’s no doubt this is a problem that requires emergency action now to see if we can avoid the worst of the tipping points. But there are many “showstoppers”, any and all of which can bring humanity and biodiversity to a sticky end.

Without biodiversity in all its forms, which creates the complex web of interrelated systems that hold the biosphere in homeostasis, things that we take for granted such as temperature, the level of oxygen in the atmosphere or the even concentration of salt in the sea, will no longer support the life we know.

Something other than climate change is driving the current mass extinction. The impacts of climate change, though potentially catastrophic, are in the main yet to come – albeit sooner than we have previously expected.

The current trajectory of biodiversity loss and ecosystem collapse is being driven by cutting down forests, over-fishing, chemical pollution, soil degradation and erosion, habitat destruction, desertification and so on. These activities are all a function of the vast amount of energy we have at our disposal. We have too much and, as we use it, we damage ecosystems.

We are well outside the reasonable limits of our energy and material use. Globally about 51% of all energy use is industrial, 20% is transport, 18% residential and 12% commercial. But all is interconnected and almost all demands over and above our basic needs lead back to increased physical and chemical destruction of ecosystems.

So what are the reasonable limits within which we can operate? I think that there are four main factors to consider.

First, according to the Global Footprint Network, we are already consuming 1.5 times more than the earth is able to renew each year. To bring our material and energy credit card back into balance we need to reduce our resource draw by 33%.

Second, world population is almost inevitably going to increase by another 25%. To accommodate these newborns, we must reduce our resource draw by a further 25%.

Third, on a scale of wealth, the top 20% of humans are 40 times richer than the bottom 20%. This is morally and geopolitically untenable. It would seem reasonable for the top 20% to reduce consumption by a factor of 4 and the bottom 20% to increase by a factor of 4. The gap between rich and poor would then be a more reasonable factor of 2.5 times. The reduction associated in resource draw for countries like Australia would be a further 75%.

Finally, we need to build in safety margins. When we build a bridge, we build in a resilience factor. According to the New South Wales government, the safety factors for the steel in the Sydney Harbour Bridge ranged from 1.9 to 2.5.

The steel chosen by Dorman, Long & Co. Ltd. for the main beams of the arch was silicon steel. In compression these beams were between 2.2 and 2.4 times stronger than the expected stress on them. Equally, we should not consider using Earth’s resources with no margin for the unexpected. If we were to choose a factor of 2 for planet Earth, then this implies a further reduction in resource draw of 50%.

Multiply these factors together and we end up with a reduction to about 6% of what we currently consume in energy and materials in Australia. That is 16 times less than we now use.

This is the goal we need to set, without which we have little chance of hitting the target – survival. I hope that survival is what most people want.

Suppose we, humanity, survive the current planetary sustainability crisis relatively unscathed and, in 100 years time, can say that we have stabilised global ecosystems. By this I mean: biodiversity has ceased its dive into mass extinction; CO2 is under control; global population is in a manageable decline; chemical and physical damage to ecosystems is being reversed and healed; and we are living within our planetary means.

To countenance such a deep and radical change requires us to rethink the entire way we organise humanity, politically, economically and socially.

I am sometimes asked where to start; I don’t think the problem is technical. It lies deep within us. At present we have adversarial systems of governance – both political and economic. These work in an open system, a world with few humans, where wounded places and populations can be abandoned for new territory.

But in a closed system, full of humanity, these wounds must be healed. Only cooperative systems of organisation, both political and economic, can achieve this.

[continued ...]

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We need to be talking about what might work and experimenting with different ideas. We don’t have long to do this, but it does offer the opportunity of an extraordinary renaissance.

http://www.psr.org/resources/climate-change-and-famine.html

Physicians for Social Responsibility in the US have an excellent resource on the projected effects of Anthropogenic Climate perturbation.

The report on Climate Change and Famine has some important findings which are well referenced to peer reviewed literature. Here is the conclusion from that report:


Quote:

Conclusion Farmers are resilient and frequently adapt to changes in weather. However, climate change will create conditions outside of human experience, challenging farmers’ ability to adapt.5 While farmers with more wealth and resources are more likely to be able to adapt to a changing climate through investments in new technologies, seed varieties and cropping patterns, poorer subsistence farmers will be less likely to adapt and are thus more vulnerable. Regardless of wealth, complete adaptation is not possible. Increasing extreme weather events has the potential to devastate infrastructure of the entire food system. Storms and flooding can destroy food processing, packaging and storage facilities and disrupt transportation infrastructure such as roads, bridges, railways, airports and shipping routes preventing available food from getting to where it is needed. Though this fact sheet focuses on agricultural crops, they are only one part of the food supply. The changing climate also affects animal production. Decreasing supply and increasing prices of feed grains will increase the price of meat.1,9 Temperature extremes will increase animal deaths and the cost of cooling animal  facilities.1,9 Rising temperatures and changing rainfall patterns will alter the distribution of animal diseases such as anthrax and blackleg, potentially reducing production.9 The overall impact on fisheries is uncertain, however a 40% catch decline is expected in the tropics as commercial species move north out of warming waters.1 Food prices will rise as climate change reduces the amount of food available. And people get angry, even violent, when food becomes more expensive.In 2008, world wheat, rice, corn and soybean prices tripled.18 Food riots erupted across Egypt, Yemen, Morocco, Cameroon, Senegal, Ethiopia, Haiti, Indonesia, Mexico, and the Philippines.18 Social order unraveled as armed Thai villagers guarded their rice fields against rice rustlers; grain trucks were hijacked in Sudanese refugee camps; Pakistani troops had to guard grain elevators and wheat trucks.18 There is concern about increased conflict and violence as food supplies constrict. Already one billion people in the world go hungry every day—that’s one in every seven people.18 Every year one third of child deaths are caused in part by under-nutrition.19 World population continues to grow and is expected to reach 9 billion by 2050.20 To feed this many people and their rising demand for animal products, overall food production must rise by 70% from 2005–07 levels.20 But a 5°F to 9°F rise in global average temperature could reduce grain yields by 30% to 50%, and global food supplies even more. The combination of decreasing food production in the face of increasing food demand would likely lead to widespread social unrest and hunger —even catastrophic global famine. This is the future that awaits us if we fail to act. But we can rewrite the future from this grim view If we work to create many solutions, from efficiencies to new renewable energy, to reduce greenhouse gas emissions by 80% by 2050. To find out how you can get involved in making a better future, visit www.psr.org to discover actions going on in your area.