I regularly see articles like this where a country has or has committed to reduce their carbon emissions by a certain percentage in relation to a certain year. That article mentions that Scotland has dropped their emissions to 49% below 1990 levels.

Other discussions talk about cutting carbon to come under a 2 degree, or 3 degree warming level.

How do these two relate?

If everyone goes back to 1990 how many degrees does the planet warm? What do different 'years' look like 2000? 1960?

edit: The answers given pretty much cover it but what I was really after was a bar graph or similar that says 1960: 0.3 degrees, 1990: 1.1 degrees, 2000: 2.0 degrees. ie how do year levels correspond to warming predictions.

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    $\begingroup$ You can find all of that in IPCC 5th report: here ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_SPM_FINAL.pdf $\endgroup$ – Gemechu Fanta Garuma Jun 12 '18 at 18:46
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    $\begingroup$ it is not possible to prevent climate change,the best we can hope for is to limit the speed of change so it stops happening faster than it does right now. $\endgroup$ – trond hansen Jun 13 '18 at 18:26
  • $\begingroup$ Your last sentence is odd. The first part is straight forward. Preventing too much temperature increase is the goal. Reduction of emissions is the means to reach that goal. That much goes hand in hand. But your final sentence is poorly constructed. Individual years vary mostly to variation in ocean currents and the occasional very large volcanic eruption. If you want to set 0.1-0.2 degrees Celsius warming per decade between 1990 and today, that wouldn't be too far off. 1960-1990 the increase would be less. I'll make this an answer if you clarify your question. $\endgroup$ – userLTK Jun 15 '18 at 9:34
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    $\begingroup$ The current wording of this question is quite unclear and confusing. I think this question tries to ask this: 'How many degrees would've the planet warmed if average carbon dioxide concentration would've stagnated in year 1990? What about 2000 or 1960?' $\endgroup$ – Communisty Jun 15 '18 at 12:11
  • $\begingroup$ The premise of your question seems to rely on the idea that CO2 emissions were special in 1990, and that is somehow related to an ideal climate. The reason they use 1990 as a reference is because it is the base year agreed on in the Kyoto Protocol. en.wikipedia.org/wiki/… So, it is a year chosen for political reasons, not scientific reasons. $\endgroup$ – farrenthorpe Jun 19 '18 at 16:22

As Gemechu Fanta Garuma commented, the IPCC Assessment Reports (at time of writing, AR5 is the latest) and its special reports are the best place to go for a comprehensive summary of the science as it was a small number of years ago.

In terms of greenhouse-gas concentrations that are stable and civilisation-friendly, the range 300-350ppm CO2 is as useful a range as we're likely to get for now - 350 is when coral bleaching starts to become a serious problem, and trashing the ocean ecosystem is a shortcut to threatening human civilisation, so we should probably avoid that. And for methane, say around 1000-1300ppb.

It's not a question of "returning to a year". We don't need a time machine, and we don't need to wind back everything that's happened. I've no wish to return to 1980, for example, and I'm not sure why anyone else would (I was there and it was largely rubbish). This is not about undoing decades of human progress. What we do need to do is to keep the concentration of greenhouse gases in the atmosphere at levels that keep the climate fairly stable at the timescales of human lifetimes, so that we avoid massive disruptions and a potential threat to human civilisation. Had we started doing that when this was first flagged up as a major issue by scientists in the 1970s, we would have done it much cheaper, much sooner, and with less collateral damage. (like in the old proverbs: an ounce of prevention is worth a pound of cure; and a stitch in time saves nine)

We have already caused some climate change. It will take 30 years for the full effects of what we've done to date, to show up. We may already have caused trillions of pounds of damage to our natural capital - but we'll only know the full scale of the damage when it's far too late. Theoretically, we could avoid some of that damage in the next 30 years by putting the global economy to work to tackle the problem seriously. However, the political will just isn't there. Most of the people who'll suffer the worst of the damage are too young to vote, or haven't been born yet. Most of the people with the ability to do something about it (voters, politicians, business leaders) will be dead before the worst of the damage hits.

The Keeling Curve shows how atmospheric CO2 concentrations have changed over time:

Keeling Curve

(chart Scrippsnews - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=69539214)

As for the relationship between atmospheric concentrations of greenhouse gases, the change in average global temperatures, and actual changes in climate, that's a set of very complex relationships wrapped in uncertainties and unknowns. And these things are generally nowhere near as simple as a neat, reversible one-to-one function: even if we were to restore greenhouse gas concentrations to where they were in 1980, does not mean that the climate will return to just as it was then. And that's why mitigation action is so very urgent: we are driving in thick fog very close to the edge of a cliff. The smart thing to do is to stop driving and work out what's going on. Which means getting to zero net emissions as fast as we can without destroying everything else in the process.

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    $\begingroup$ Might be worth noting the residence time of CO2 in the atmosphere estimated in AR5 Chapter 6, that we're still talking about CO2 above baseline levels for ~1,000 years. Even emissions at 1960 levels would still be adding to the excess CO2 levels, albeit more slowly, which is why organizations like the University of California encourage emissions reductions aimed at "bending the curve" of projected warming downward (climatechampions.ucop.edu/2017/02/08/what-is-bending-curve) to allow more time for adaptation and remediation. $\endgroup$ – jeffronicus Jun 15 '18 at 13:43

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