Estimate 21st Century Global Temperature Trend

[WeatherRusty]

Total global surface warming since ~1750 has been about 0.9C as a consequence of all combined external forcing, feedback and factors of internal variability. Given your present understanding of the physical basis for AGW and other factors which may have and will continue to influence global average temperature, what do you expect will be the remaining net temperature response by mid to late 21st century since about 1750, the last time CO2 concentration was at it's natural background level of ~280ppm?

CO2 concentration certainly will have doubled to ~560ppm by sometime mid 21st century.

Could you explain your reasoning?

[skierinvermont]

FWIW the IPCC A1B scenario is for 2.8 of warming from 1980-2000 base, which would be about 2.5C from present (2005-2015 base) with a likely range of 1.4-4.1.

My personal opinion is we will see about 1.5-2C more warming from present to the end of the century, which would be on the low end of the A1B scenario likely range. I think this

1) because I think emissions will be slightly less than the A1B scenario and

2) I lean towards the lower end of the climate sensitivity spectrum because given the large amount of uncertainty it is prudent to give some weight to a linear rather than exponential trend. This isn't to say I would ever suggest less warming than the physics dictates at a minimum. There is very high confidence that an A1B scenario would lead to over 1.4C of warming. So while modelling suggests 1.4-4.1 (maybe 1.1-3.5 on a lower emissions scenario) usually it is wise to hedge towards the lower end of this in making a prediction. Also we do seem to be a little under the IPCC projections at present although this could be related to the recent solar minimum.

[PhillipS]

My prediction of around a 2 C rise by the end of this century is a compromise between my optimistic and pessimistic sides. The optimist in me hopes we'll get a handle on CO2 emissions and at least stabilize at a level we can adapt to. The pessimist in me fears that we will stay addicted to fossil fuels until a crisis forces a change. And that change will not be pretty.

Rusty is correct that under BAU we will double CO2 by mid-century. If the IPCC estimate of climate sensitivity is correct we (or our descendants) will see about a 3.5 C total rise in global temps. But due to the thermal inertia of the oceans not all of that will be realized by 2100. So a 2 C rise over what we've already observed is plausible in my opinion.

[okie333]

Fall around 0.5 (though I think it will be more, I'm not sure if it will continue through the end of the century)

[WeatherRusty]

I am going with the lower end of the IPCC estimate or 1.1C - 2.0C of additional warming during this century. The upper end of that range encompasses the 2.7C determined most likely to represent equilibrium climate sensitivity for a doubling of CO2 according to the literature the IPCC relies on for it's findings.

Of course, climate sensitivity does not imply an expectation the temperature will rise a total of 2.7C, as other factors are in play in the real world which complicate matters. How much CO2 will we actually dump into the atmosphere? Will the CO2 sinks remain as efficient as they currently are. Will methane out-gas at increasing rates from a thawing arctic tundra? How will future aerosol pollution impact on temperature? How will cloud amount be affected if at all by galactic cosmic rays? At what rate will tropical deforestation take place? And so on.

I don't expect PDO to make any contribution, except to periodically speed up and slow down the general upward trend in global temperature. The same goes for any other case of internal variability which does little to affect the top of atmosphere energy imbalance averaged over decades.

Because of the inherent uncertainty apparent in the factors listed above and others not mentioned, I find the full IPCC range to be a distinct possibility, as the evidence we have indicates how past climates would have reacted to similar forcing we find today. That is not to say a climate response outside that range is not possible, but rather becomes less likely.

If the next several decades progress in a similar manor as in the past then the most rapid warming will not occur as there will be periods when the rate of warming slows due to natural variability. So, through this century I don't expect the rate of warming to average more than 0.2C/decade, which would likely place the total additional rise in temp to between 1.1C and 2.0C.

[superjames1992]

1.1-2.0C is my guess. I don't think I'll live to know whether it verified, though (or maybe I will?).