How accurately can we predict the climate consequences
of carbon emissions?
How big is the uncertainty?
There is no simple answer, but the uncertainty
varies greatly over the four steps in the carbon climate chain.
Current emissions are known with less than 15% error.
The uncertainty in future emissions is immense,
as it depends on assumptions about what the world will
look like a century from now.
How many people?
How rich?
How will they get their energy?
How much will they care about environment?
A casual look at a 19th century forecast of the present day
reveals the deep, enormous uncertainty in any attempt
to predict the far future.
But the real question is not to passively predict the future--
who's the client for that?-- but rather to ask what kind of future we want
and aim there, understanding the implications of our choices
for our children's future and understanding that our children may
well have very different ideas about the future they want than we do.
Atmospheric concentrations are known with very high accuracy.
The concentration of CO2 in the atmosphere
is known to much better than 1%.
Scientists understand the carbon budget in great detail,
fitting together different pieces of science
to tell a coherent, well-understood story about where carbon comes from
and where it goes.
So the overall uncertainty in predicting CO2 concentrations in, say,
the year 2100, is small, about 30% if we knew the emissions perfectly.
It's much harder to predict the amount of climate change
for a given increase in carbon dioxide concentration.
Here the overall uncertainty is perhaps a factor of two or even three.
That is, if CO2 concentrations are doubled,
temperatures might rise anywhere from one degree, a quite small climate
change, to six degrees, a really monstrous climate change,
though there is better than 2/3 chance that the temperature
rise would be something between one and 1/2 and four and 1/2 degrees
centigrade.
Finally, it's harder still to predict many of the impacts of climate change
on humans or the natural world.
Predicting the impact of climate change on regional productivity
of crops like rice or wheat is uncertain enough
if we had to assume that agriculture and technology practice stays constant.
But that's the one thing we can be sure it will not do over the next century.
Farmers will change crop varieties and practices
to adapt to a changing climate.
So the uncertainty is big for all the steps.
And worse still, the uncertainty cascades from step to step.
So can we say anything useful about the impacts of climate change in 2100?
The answer is yes.
This roulette wheel illustrates how the future temperatures might look
if we do nothing to restrain emissions.
We could get lucky and have very modest climate change in which
many people might even benefit.
Or with equally small odds, we might get unlucky and have very rapid climate
change that was bad almost everywhere.
More likely the answer would be somewhere in the middle.
Now this second wheel shows the results of a specific climate policy.
The climate changes are much smaller.
The probability has shifted to a much better outcome.
There is little doubt about which world we'd rather live in.
We'd like to have less chance of damaging climate change.
But of course, that leaves out the question
of what it costs to cut emissions and who pays.
Some people argue that we should ignore climate change because our models are
poor and the uncertainty is huge.
The uncertainty is huge, but that's not a convincing argument for inaction.
Uncertainty cuts both ways.
Reality could be better or worse than our best guess.
It's bad policy to simply ignore the massive changes implied
in the low probability, high impact segment of the roulette wheel.
The hard question is not, should we do anything, but rather how much should
we do?
Who pays?
How do we coordinate our actions around the world to achieve a shared goal?
This question has no single objective answer.
Values and beliefs enter in a central way.
In forming your own answer to that question,
you have to figure out your own view on the basic question,
how much does climate change matter?
How might it affect our lives or the lives of our great grandchildren?
And how does it compare to the many other challenges humanity faces?
Not all climate changes will be bad for all people.
People in northern latitudes may benefit as a growing
seasons lengthen for important crops.
Industries that want to exploit resources in the Arctic
may benefit from the nearly ice-free conditions in the summertime.
But there is little doubt that most people in the natural world
will be worse off if climate changes fast.
And it's a certainty that if humans in this century
let CO2 concentrations get to, say, three times their pre-industrial value,
they will deprive future generations of a chance
to enjoy much of the natural world as it was at the start of the Industrial
Revolution.