> The atmosphere is pretty tightly coupled to the ocean, especially the
> surface. I suppose it's possible that some altitudes would be affected
> more significantly, depending on the details of aerosol behaviour (a
> point that I'm not at all expert on).
I've been thinking quite a bit about thermal lag, it's an area that I
like, because of my background as a chemical engineer.
A good comparison ought to be seasonal changes. Over a day forcing
changes by about 1000 W/m2, over a season I don't know exactly, order
of magnitude wise it ought to be around 200 W/m2.
In Siberia, seasonal temperature changes I think approach 50C, in
England it's more like 20C.
Some of the forcing will go towards heating soil and air, some will be
compensated by more radiation.
Aerosols are currently around 1 W/m2 and with climate sensitivity of
3C that's equivalent to around 1C without thermal lag.
Currently, around 0.5 W/m2 or so go towards heating the oceans, 1 W/m2
is offset by aerosols (0-2.5) and 2.5 W/m2 is greenhouse gase
forcing.
I think this gives me some reasonable bounds for the response to a
sudden decrease in aerosol forcing. There ought be a jump of at least
0.1C over the first year, but more likely 0.5C.
The key here is looking at the ratio of the seasonal temperature
change and the seasonal forcing change. If there was no thermal
inertia (and no heat transfer from the tropics), 200 W/m2 should mean
roughly 200C with a climate sensitivity of 3C. When it is actually
50C, 3/4 of the forcing go toward heating air and soil.
The longer term bound seems to be about a third going to heating the
oceans, so 0.5 C after the first year, 0.7 C after 2-3 years and 1C
after decades to a century ought to be the response to a step change
in forcing by 1 W/m2 (roughly, roughly)?
---------------------
I find it quite helpful to look at the heat capacity of air and
oceans. 10 km of air weighs about as much as 10 m of water. The heat
capacity of water is 4.2 kJ/kgK, the heat capacity of air 1 kJ/kgK.
How long does it take to heat 10 km of air at 1 kW/m2 (full noon time
insolation)? 10 km times a metre is 10,000 m3 or about 10,000 kg. So
we need 10,000 kJ or a 1 kW, 10,000 second, or just under 3 hours.
Day/night time differences are 10C or higher and it doesn't take 30
hours to go from the daytime high to the daytime minimum, but that's
because only a fraction of the atmosphere actually goes through the
temperature change (the bottom most layer, temperature higher up is
much more constant).
How long does it take to heat 1 km of ocean at 0.5 W/m2 (current heat
take up by the oceans, roughly)? 1 km of ocean weighs 1,000,000 kg and
has a heat capacity of 4.2 billion J/K. At 0.5 W/m2, heating the ocean
by 1 K will therefore take 8.2 billion seconds, or 260 years.
> Well unless one postulates an extremely high sensitivity to such forcing
> (but not to other forcings), one would expect the warming to be
> levelling off - and there's no sign of that except through some rather
> specious cherry-picking vis a vis 1998...
I suppose, playing devil's advocate here, the recent lack of levelling
off might be due to say the declining aerosol forcing Michael Tobis
mentions ("the trend in aerosol emissions from Chinese power plants is
sharply downward").
Not being a climate scientist I am a little confused about what the
concept of forcing really means in the case of cosmic rays. My
understanding is that the forcing is measured in W/m2 at the ground,
assuming no feedbacks from water vapour, clouds etc.
So, say for CO2 we have say 2W/m2 out of 200 W/m2 (or actually 350 or
so, when taking into account greenhouse gases), simple radiation is
based on the fourth power of temperature, so from 291 to 293 K we are
talking an increase of just under 3% and that's not 2 W/m2, it's more
like 10. The difference is water vapour, clouds and other feedbacks
(that are operative on medium scales, or in other words, ice sheet
extent changes, or changes in surface albedo due to different plants
growing there are specifically excluded).
This is fine for a forcing like CO2, but when the forcing affects the
feedbacks it wouldn't be.
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