by Judith Curry
Previous IPCC reports — and much of the debate over how to react to them — have appeared to treat the Earth’s climate as if it were a domestic central heating system, with carbon emissions analogous to the dial on the thermostat: a small tweak here will result in a temperature rise of precisely 0.2°C and so on. – The Spectator
The Spectator has a really good piece entitled Finally, the IPCC has toned down its climate change alarm. Can rational discussion now begin? The whole article is well worth reading, but here I want to focus on the above quote.
This issue of CO2 as a climate control knob has always bugged me, I’m not sure how/what to think about this. Several years ago Andy Lacis did a post at Climate Etc. discussing his paper Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature. The paper argued that Without the radiative forcing supplied by CO2 and the other non-condensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate to an icebound Earth state.
From the perspective of comparative planetology, I think that Lacis makes a plausible argument, from which I infer that without CO2 in the atmosphere, the Earth’s climate would more closely resemble the climate of its moon rather than the current Earth’s climate. For reference, the surface temperature of the moon can swing between -150°C during the night and 120°C during the day. (pay attention, greenhouse effect deniers).
But on what time scales does it make sense to think of CO2 as a control knob? That is a very relevant question in context of CO2 mitigation policies that doesn’t seem to get asked. The conventional (IPCC) ‘wisdom’ has been that there is a time lag in the climate system of nominally 50 years, after which time the impacts of different emission scenarios would be felt on the Earth’s climate (surface temperature, sea level rise, extreme weather, and all that).
The time lag in the system is a conceptually useful but unfortunately overly simplistic way of thinking about this. I think a better way to think about all this is in the context of feedbacks on different time scales. Most of the reasoning about climate feedbacks is in the context of the fast atmospheric feedback processes (e.g. water vapor, clouds, lapse rate), and the slightly slower feedbacks associated with sea ice and seasonal snow cover. The feedbacks on glacial time scales are obvious from plots of time series of CO2 concentration and inferred surface temperature, whereby surface temperature variations leads the variations in CO2. On geological time scales, there are presumably a whole host of other relevant processes, not to mention the sequestration of carbon into fossilized organic materials (otherwise known as fossil fuels).
So, how should we think about all this on decadal to century time scales, which are the timescales of most relevance for CO2 mitigation policy making? Well, the ‘pause’ should give pause to anyone who thinks that CO2 controls temperature/climate/weather on the time scale of a decade. What about multi-decadal to century timescales? On these time scales, the big issue is the natural (unforced) internal variability. Two recent guest posts at Climate Etc. provide insights here:
- What is internal variability? – X Anonymous
- Unforced variability and the global warming slow down – Patrick Brown
If the climate is dominated by internal variability on these timescales, then external forcing causes only a small deviation in the climate, and the climate on these timescales is on a very long ‘leash’ with regards to external forcing.
For me, the fundamental and most burning question in climate change dynamics is the unforced internal variability, and its relative importance to external forcing and its interplay with the external forcing. The IPCC all but dismisses the importance of natural internal variability in its explanation of 20th century climate attribution, as it provides an explanation of 20th century climate variability almost. Political pressure to explain the pause is forcing them to pay some attention to natural variability.
Until we get get past the IPCC’s paradigm paralysis that climate change on multidecadal time scales is completely externally forced, there won’t be much progress on really understanding climate change.
The following figure was sent to me by Roger Pielke Jr: