Former U.S. Secretary of Defense Donald Rumsfeld is famous for his statement that there are “known knowns,” “known unknowns,” and “unknown unknowns,” and it’s a useful framework for thinking about what we do and don’t know about the future of climate change.

While there’s a lot we can say with a great deal of confidence when it comes to future climate change, there is also A LOT of remaining uncertainty, particularly when it comes to near- and mid-term outcomes, the timeframes likely to be of most concern to many decision-makers. 

We’ll briefly explore these three categories, and you can then dig deeper into specific sources of uncertainty. 

What Do We Know for Sure? (Known Knowns)

After decades of intense scientific research we know a lot. About how the atmosphere works, about the history of the earth's climate, and how human activities are influencing the earth's climate today. For example:

• We know that Earth only supports life because trace levels of GHGs (measured in parts per million) naturally warm the atmosphere by approximately 15°C through the so-called ‘greenhouse effect’.

• We know that global temperatures have varied substantially over tens of millions of years, but that temperatures during the last 10,000 years have been remarkably stable within a 1^o C band). Agricultural and all other human systems have evolved within those bounds, and the human population has gone from millions to billions.  

• We know that human activities are changing the composition of the atmosphere. The pre-industrial CO₂ concentration of ~278 ppm (parts per million) has grown to more than 410 ppm today. This is amplifying the natural “greenhouse effect” and leading to climate heating

• We know that the last time global temperatures were 2°C and 3°C higher than in 1900, global sea levels were 4–6 meters and 20–30 meters higher than they are today, respectively.

• We know that ocean acidification resulting from the buildup of CO₂ in the atmosphere is interfering with shell formation and crustacean reproduction, that it will have widespread impacts on fisheries and livelihoods, and is likely to lead to the loss of most of the world’s coral reefs even within decades.

• We know that fully consuming today’s conventional fossil fuel reserves could increase CO₂ concentrations to well over 1000 ppm; taking advantage of all unconventional fossil fuel resources would add dramatically to that number.

• We know that a host of observed changes to local and global biological and other systems are consistent with expectations of a higher-CO₂ and a warmer world. 

Known Unknowns of Climate Change

Here are a few of the "known unknowns" of climate change.  

• We don't know the extent to which the current rate of "climate forcing," which is much higher than during past cases of “natural” climate changes, will manifest as changes in how the climate reacts to that forcing. By one estimate today's "climate forcing" is 100-200 times what it was during earth's prior history. That makes it hard to know whether climate models, which do a good job of recreating climate history, will end up accurately forecasting the path of future climate change (as opposed to the eventual equilibrium).  

• While we know that a warming world will influence the formation of clouds and the extent of cloud cover, we don't know for sure how. The "cloud feedback" could be positive or negative depending on the answer. 

• We don’t know for sure whether the biosphere will continue to absorb approximately 50% of the CO₂ annually emitted by human activities, or whether that fraction might fall if the oceans or forests as natural sinks become CO₂ -saturated, or if natural sinks like forests turn into sources due to climate change itself (e.g. expanded forest fires).

• We don’t know for sure how GHG emissions will change over time, since those emissions depend on population growth, economic activity, energy prices, public policies, and other variables.

• We don’t know for sure where CO₂ concentrations will get to during this century, whether 450, 550, or even more than 650 ppm. 

• We don’t know for sure how sensitive the atmosphere will prove to be to rising GHG concentrations. Most climate modeling assumes that a doubling of CO₂ concentrations will translate into approximately 3°C of average global temperature rise, but estimates range up to 6^o C. 

• We don’t know for sure the many ways in which climate change will manifest through 2nd and 3rd order effects, partially because models can’t forecast those effects.  

• We don’t know for sure the degree to which individual events reflect the impact of climate change as opposed to natural climatic variability, although our ability to attribute events (or at least the probability of such events) is arguably increasing over time. 

• We don’t know how quickly sea levels might rise, given that historically each 1^o C of average global temperature change has been associated with 20 m of average sea level rise. 

• We don’t know whether and when a range of tipping points might trigger positive feedback loops and accelerated climate change.

• We don’t know when we’ll witness systemic climate risk events resulting from climate change forcing, and the consequences of those events. 

• We don't know whether an effective social movement will develop around climate change, and with what timing and effect. 

• We don’t know whether an organized “resistance” movement will develop to interfere with and even attack fossil fuel infrastructure. 

• We don’t know for sure how long (at least in the US) national climate policy will be blocked, and what might come next? 

• We don’t know how badly economic models are at estimating the future economic costs of climate change, or the economics of mitigating climate change.

• We don’t know whether companies might eventually be found liable for GHG emissions. 

• We don’t know whether and when consumers might start acting on their climate change beliefs in ways that would generate significant brand risk at the sectoral or company levels.  

Unknown Unknowns of Climate Change?

Enumerating the “unknown unknowns” of climate change obviously poses a challenge, since that would require that we know something about them! The approach we take below is to identify “climate wild cards” that we’ll consider a plausible proxy for "unknown unknowns." Scientists are talking about these wild cards, but there is no consensus about their impacts.

• What if climate models are significantly under-estimating future rates of climate change as a result of the workings of some of the “known unknowns” above?

• What if we start to see rapid large-scale releases of methane from melting permafrost or from methane clathrates on the ocean floor?

• What if changing ice sheet dynamics in Greenland and Antarctica cause much more sea level rise than conventional estimates have suggested by 2050 and 2100?

• What if the Atlantic Oscillation ocean current suddenly slows, with huge implications for Europe (at the very least)?

• What if climate change combined with natural variability leads to simultaneous crop failures around the world, resulting in global price shocks and civil unrest?

• What if climate ‘tipping points’ lead to sudden step changes in observed climate change?  Hypothesized tipping points including dieback of the Amazon forest, slowing of the Atlantic ocean current that warms Europe, melting of the permafrost, among other variables as shown in the graphic below. If one or more of these tipping points are triggered it could have a significant impact on climate forecasts. 

• What if the triggering of "climate tipping points" causes a draconian policy backlash with particularly disruptive business implications. This is the idea described in Paul Gilding's "The Great Disruption."  (2010)  

• What if climate change accelerates in ways not currently part of the “consensus science” forecast, resulting in physical and other impacts much sooner than expected. 

With literally dozens of climatic and socioeconomic variables in play when it comes to forecasting future climate outcomes, robust decision-making around managing uncertainty and risk is uniquely challenging.