Tuesday, September 21, 2010

helpful climate change overview

It is not as sexy to be a doubtist (white flag) rather than an alarmist (green flag) or denier (red flag). Doubt.

But quite possibly climate scientist Judith Curry presents a superior overview of the science of climate change than the way IPCC findings are presented to us. I found this exchange (starts here) on her blog helpful:

Bart Verheggen:
My major issue though is this:

For the remainder of this century, what natural forcing or variability could plausibly rival the relentlessly rising anthropogenic forcing in magnitude? Is there evidence at all for that being plausible? If so, is that evidence really as large as the evidence showing that greenhouse gas forcing will exceed the likely bounds of natural variability (if it hasn’t already)? Or alternatively, so you really believe that an equal portion of the climate change over the next 90 years will be caused by natural variability versus caused by natural variability/forcing?

I haven’t seen any plausible evidence for such.
Judith Curry:
Bart, the issue for the 21st century is this. NOBODY in the IPCC has tried to actually predict 21st century climate change. What they have done is conduct scenario simulations for adding greenhouse gases into the atmosphere over the next century. They do NOT predict 21st century solar variability or volcanic eruptions. They do a poor job at simulating the observed modes of natural internal climate variability (e.g. the multidecadal ocean oscillations).

So what the IPCC simulations basically say is that, if the solar and volcanic forcing remains fairly neutral in the 21st century, then CO2 warming will dominate and they provide specific projections for these scenarios. The elephant in the room is that no one is predicting the natural variability for the 21st century (we don’t know how to do it, basically).

So even if we knew the CO2 sensitivity perfectly (which we don’t), we don’t know how to estimate the natural variability piece, which could be smaller, equal to, or larger in magnitude than the greenhouse forcing. If equal to or larger in magnitude, then during some periods greenhouse warming will be cancelled out by natural variability and in other periods greenhouse warming would be the same sign as the natural variability.

We already know what the natural variability looked like in the 20th century, no big surprises but still an unexplained increase between 1910-1940 and decrease between 1940 and 1970. We have no idea what 21st century natural variability will look like, but already we are seeing surprises from the sun re sunspots. So this is why I bumped up the size of the white for the 21st century.
and in a follow up comment from Judith:
IMO, too much emphasis and focus has been given to greenhouse forcing, and insufficient focus on natural variability and land use changes
The two candidates (apart from volcanic forcing) are solar variability and the natural internal variability of the coupled ocean atmosphere system, e.g. the multi-decadal and longer oscillations such as the NAO, PDO, etc. Not to mention abrupt climate change, which has been documented in the past to occur without any obvious external forcing.
NAO = North Atlantic Oscillation
PDO = Pacific Decadal Oscillation

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