What Are the “Unknown Unknowns” of Global Warming

Year after year the worriers and fretters would come to me with awful predictions of the outbreak of war. I denied it each time. I was only wrong twice.

— Senior British intelligence official, retiring in 1950 after 47 years of service

This weekend’s question is: What are the “Unknown Unknown” climate impacts?

In 2002, Former Defense Secretary Donald Rumsfeld infamously popularized the term “unknown unknowns” — “the ones we don’t know we don’t know.” See video below.


As my illustrated review of 50 recent studies on climate impacts made clear, what we know with confidence is coming on our current emissions path is more than enough reason to act.

If we go to 7°F — let alone 9°F or higher — we are far outside the bounds of simple linear projection. Some of the worst impacts may not be obvious — and there may be unexpected negative synergies. The best evidence that will happen with the staggering warming we face if we keep doing nothing is that it already happened with even the 1°F or so warming we have seen to date.

As quantified in the journal Nature, “Mountain pine beetle and forest carbon feedback to climate change,” (subs. req’d), which just looks at the current and future impact from the beetle’s warming-driven devastation in British Columbia: “The cumulative impact of the beetle … converted the forest from a small net carbon sink to a large net carbon source.

No wonder the carbon sinks are saturating faster than we thought (see here) — unmodeled impacts of climate change are destroying them:

Insect outbreaks such as this represent an important mechanism by which climate change may undermine the ability of northern forests to take up and store atmospheric carbon, and such impacts should be accounted for in large-scale modelling analyses.

And the bark beetle is slamming the Western U.S. and Alaska, too.

The key point is this catastrophic climate change impact and its carbon-cycle feedback were not foreseen even a decade ago — which suggests future climate impacts will bring other equally unpleasant surprises, especially as we continue on our path of no resistance.


Note I am not talking about the many “known unknowns” — the stuff we know could happen but we have no idea how fast and fierce:

I’m talking about stuff that is not really explored in the scientific literature.

I’m talking the real black swans — an “extreme event” that is “an outlier, as it lies outside the realm of regular expectations, because nothing in the past can convincingly point to its possibility” (see “Is global warming a black swan”).

What are the “Unknown Unknown” climate impacts?