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There are 6 billion people in a highly complex barely feeding themselves dependency on agriculture, all evolved for the temperatures and rainfall patterns and weather patterns that prevailed over the past 10 millennia. A pretty limited range and it would be stretching it to say we're feeding everyone. Plus 4 is a small change in weather but a MASSIVE change in climate. The Glaciation to here is only about 6 degrees IIRC.

At Plus 4 much of southern Europe, north Africa and the Middle East become uninhabitable - climate refugees move North (South in the Southern Hemisphere). (My personal expectation is a MINIMUM - 1.5-1.7 meters of sea level rise by 2100 but THAT does NOT stop for a thousand years).


The best case is a smooth transition to the steady state. We aren't so good with the transients. What we've done is put in a ridiculous step function forcing change. Temperature is rising 20x faster than its wont, and our CO2 step function is 50x faster. Rapidly changing weather is NOT good for agriculture. Neither are the steady state heat and drought and major flooding extremes that we get in the smoother case... the combination is almost certain, at plus 4, to cut agricultural productivity to less than half... globally. It will mostly be poor people who starve... but some own weapons of mass destruction and they WILL be desperate... they WILL blame the USA. (with some reason I think)

History contains many ruins of civilizations that suffered some form of overshoot of their natural environment's resources of food and water. What we are doing is different. We are ALTERING those resources through our manipulation of the climate. We are doing this without intent and in the case of people who ignore scientists, without knowledge but even if we pulled back on our population and did not ever overshoot our current food production, we still wind up in overshoot when that production crashes back. The water shortages, energy shortages and heat stress will put paid to the "green revolution" that was fueled by plenty of energy and water.

This change will (I think) come more suddenly due to the step function of the increase. It may indeed, be too late to prevent it at all. The planet is (my best guess) in climate terms, going to ring like a bell before it settles down with each reverberation spanning a human generation.


Feedbacks - The most likely being the release of Carbon from the permafrost. We are pretty sure the Methane Hydrates will stay put. We must hope that they do. The permafrost will amplify the effects of our emissions. Other feedbacks are also possible several have been identified and none can be easily discounted. What this adds for us is a fairly strong expectations that we are going to trigger some of these effects, making it harder to stabilize temperature. This is not directly, an impact on our civilization. What it becomes though, is a challenge, a demand to cut emissions much more strongly than previously thought necessary. At 4 degrees this will no longer be a matter of reasoned discourse. At 4 degrees the effort to not reach 5 will be more like a religious jihad.


Much of our infrastructure, much of our ENERGY infrastructure is located on coastlines and rivers to obtain cooling water. We have seen some of the risk at Fukushima... all such plants will have to be abandoned and relocated as the water rises. Many will have their output cut as the water warms or river flows fall below minimums. Our civilization RELIES on energy.

Consider too what happened to our supply of disk drives with a single flood in Thailand... the rise and fall of water will become larger and some (much?) of our industry will be found to be more fragile than expected.

We have been living a great economic lie by using money that does not represent work done, and when the energy disappears, an awful lot of our civilization goes with it.


Individually these things would be bad enough. Cumulatively I expect them to basically end anything that resembles a "global" civilization. Global trade will be non-existent. Resource wars will be short, sharp and genocidal. A lot of things will be broken... the internet for one... and while it is possible that pockets of civilization will survive, the reach of our species will be severely shortened. Part of the problem is that to reach plus four we'll have committed a pretty substantial error and our chances of stopping are much poorer, due to the feedbacks we expect. The other part is that that temperature regime takes us back 50 million years, but things were very different then and we can't actually look to that time to understand what the CLIMATE will be now (Continents in different places)... and we are headed for that same temperature in less than 300 years, not 50 million. The transient conditions are apt to be deadly.

I am not alone in this assessment. Most scientists recommend plus 2 as the maximum "safe" limit, as beyond that the risk of feedbacks rises exponentially. Like many, I no longer believe we'll manage to stay under plus 3 any more than we could avoid going over the fiscal cliff.
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