Long-term ENSO forcing.

Bob Tisdale has been suggesting for quite some time that the observed rise in global surface temperatures is simply the result of ENSO cycles. Many have pointed out that ENSO cycles simply move energy around, so how can they lead to a long-term rise in global surface temperatures. In a recent comment on a WUWT post, Bob Tisdale appears to agree that ENSO is not a forcing, but does claim that it can lead to long-term variations in global temperatures.

I wanted to check if there was any possibility that this could be correct. Firstly, I have to assume that Bob Tisdale is suggesting that the 40% rise in CO2 levels in the atmosphere in the last 100 years or so has not had any significant effect. The basic argument seems to be that there is a lot of energy in the oceans and so ENSO cycles can release this energy and this energy can lead to an increase in surface temperatures. I have to assume that his idea implies that the composition of the atmosphere has not influenced the Earth’s equilibrium temperature for the last century or so. There is some evidence, however, that the Solar flux has increased by about 0.1% in the last few hundred years. One can estimate the equilibrium temperature using
σ Teq4= 1/4 (1 – A) F,
where F is the Solar flux and A is the albedo (about 0.3). A 0.1% increase in Solar flux, increases the equilibrium temperature by about 0.05oC. Admittedly, the above equation gives the top of the atmosphere equilibrium temperature. If, however, greenhouse gas increases have not influenced our equilibrium temperature, the gradient should be unchanged and hence a 0.05oC increase at the top, should produce a 0.05oC increase at the surface (actually I think this is slightly wrong because the rate of change of flux with temperature depends on temperature, so to balance the flux requires a smaller temperature change at the surface than at the top of the atmosphere – same conclusion though).

Okay, so we’ve observed about a 1oC increase in surface temperatures since the late 1800s. The above suggests that this can’t be simply because of increases in Solar flux. Bob Tisdale suggests that this is due to ENSO cycles bringing energy from the deep ocean. Could this be possible? The atmosphere has a mass of 5 x 1018 kg, and a specific heat capacity of about 1000 J kg-1 K-1 and so a 1 K (or 1oC) increase would require 5 x 1021 J. This is actually quite possible. The oceans contain a lot of energy.

Surface temperatures appear, however, to have gone from amongst the lowest of the last few thousand years to amongst the highest. Presumably that implies (if Bob Tisdale’s idea has merit) that the temperature must have gone from below the equilibrium temperature to above the equilibrium. Let’s assume that the surface is now 0.5oC above the equilibrium temperature. This would mean that we are now radiating more energy into space than we receive from the Sun. The excess can be calculated using
Δ L = 4π RE2 σ [(Teq + ΔT)4 – Teq4].
If Teq=290K and ΔT = 0.5K, then the excess is 1.4 x 1015 J s-1. If the surface temperature is 0.5oC above equilibrium, then the atmosphere would have about 2.5 x 1021 J of excess energy. At the rate just calculated, this energy could be lost in less than a month. Even if we’re only 0.1oC above equilibrium, it would still only take a few months.

So, this doesn’t really make sense to me. Although it seems that ENSO cycles can indeed bring energy from the oceans to heat the land and atmosphere, any excess energy should be radiated back into space on very short timescales. The surface temperature should drop back to equilibrium very quickly. Possibly, however, the ENSO cycles are bringing us back to equilibrium from a temperature that is below the equilibrium temperature. Somehow (for reasons as yet unknown) we ended up about 1oC below the equilibrium temperature in the late 1800s and ENSO cycles have been bringing energy up from the oceans to heat the land and atmosphere and increase the surface temperature back to equilibrium. If the surface temperature is 1oC below equilibrium, then we can use the above equation to calculate that we’d be receiving 2.8 x 1015 J of excess energy every second. To increase the atmospheric temperature by 1oC would require 5 x 1021 J and hence this could be done in less than a month, given this excess energy input. Some of this excess energy will go into the oceans, so not all will be available to heat the land and atmosphere. However, if this energy is to heat the land and atmosphere at a rate slower than 1oC per century, more than 99.9% of this excess energy must go into the oceans. This again seems unrealistic. Land makes up 30% of the Earth’s surface so what process could act to prevent more than 0.1% of the incoming excess energy from heating the land and atmosphere.

This post has got rather convoluted, but the point I was simply trying to make is that it seems that Bob Tisdale’s idea doesn’t work unless some other process has been changing the Earth’s equilibrium temperature at the same time as ENSO cycles have been bringing energy up from the ocean to heat the land and atmosphere. If not, the surface temperature should have remained close to equilibrium because any excess energy should either have quickly heated it up to equilibrium (if it was below) or been lost into space and quickly brought the temperature back to equilibrium (if it was above).

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