Paris climate talks : Five things climate scientists do actually disagree about

December 11, 2015 12:21 pm

 The climate talks are underway, so we should be prepared for
dissenters to attack the process. “Climate isn’t settled” is a
favourite call and a favourite excuse for inaction on global warming.
It’s therefore worth noting that a vast majority of real climate
scientists agree on the existence, causes, and approximate magnitude of
man-made . Not every aspect of climate science is
completely resolved however, and here are five questions that are
guaranteed to get the experts going.

 It’s still not easy to determine exactly how much climate change has been due to human activities and how much is natural.

1. Do clouds intensify climate change?
Clouds
are tricky because they are patchy, and thus hard to model, and they
have strong effects, both cooling (by day) and warming (by night).
What’s more, those effects vary with cloud type, altitude, latitude or
time of year.
To make it worse, what we really want to know is
not just how big their net effect is, but how it will change as the
climate changes – what is known as a feedback effect. The best estimates
we have right now suggest the effect is fairly small, but positive.
This means clouds amplify any changes in the climate, however caused,
making the overall system rather more sensitive to man-made
interference.
It’s something we’d really like to understand much
better though, and clouds have been top of the list of uncertainties in
climate science for decades.

2. Sea levels are rising – but how fast?

We expect the sea level to rise as the ocean warms, just by regular thermal expansion.
We expect the sea level to rise as the ocean warms, just by regular thermal expansion.
We expect the sea level to rise as the ocean warms, just by
regular thermal expansion. That’s the easy bit – and it won’t be all
that much, or very fast. But, more significantly, the sea level will
eventually also rise increasingly rapidly as land-based ice sheets melt
(sea-ice floats and so has no effect if it melts, as Archimedes realised
in his bath).
Curiously, we know rather well how much water is
locked up in the ice-sheets and so how much the sea level would rise if
much of it melted. It’s a lot: it could easily be 10m or more. What we
don’t know at all well is how fast it’s likely to happen. It matters a
lot to us whether it’s centimetres per century or metres per century –
and it could be anywhere in between.
3. Should we be worried about carbon in soil?

Plants draw carbon dioxide out of the atmosphere as they photosynthesise, and release it again as they respire or die.
Plants draw carbon dioxide out of the atmosphere as they photosynthesise, and release it again as they respire or die.
The biological carbon cycle is another example of a climate
feedback effect where any change makes it warmer (or cooler) as it
warms, and vice versa. Plants draw carbon dioxide out of the atmosphere
as they photosynthesise, and release it again as they respire or die.
This operates both on land and in the sea, and respiration in both is
known to be affected by temperature, but it’s less well understood than
we would like.
There are very big reservoirs of carbon in both
soils and in the oceans, so if they were released by warming more
quickly than we think, our projections would be off. Methane locked up
in permafrost has been a particular worry, but at present it looks
likely that this will be released quite slowly. The ultimate size of
these effects is still a moving target, though.
4. Will oceans keep absorbing carbon dioxide?
We
know the oceans are absorbing most of the extra heat from global
warming, and much of the extra carbon dioxide that has caused it.
However they only do so rather slowly, because the oceans are very deep
and both heat and CO2 take a long time to penetrate beneath the surface.
Mixing between surface water and the ocean depths is assisted by global
“conveyor” currents, but we have good evidence these have varied in the
past.
Will climate change cause further variations in future?
And if so, by how much? It’s not yet known for sure, because we have too
few observations to pin down the models, which disagree about this.
We’ll probably see a gradual slow-down rather than the sort of shut-down
“tipping point” seen in the movie The Day After Tomorrow – but we still
can’t be certain.
5. Just how much are we responsible for all this?
It’s still not easy to determine exactly how much climate change has been due to human activities and how much is natural.
However,
clever statistical attribution studies have analysed the “fingerprints”
of various processes that may contribute – and these now unambiguously
give the answer “most of it”. That’s a sufficient basis for taking
action, and getting a more accurate answer would not change the outcome
significantly. But it would still be nice to know.
These
still-to-be-settled issues are some of the main contributors to the
uncertainty in our projections for the future. All those processes are
included in present-day climate models, as well as we know how: refining
them could shift the projections a bit either way, but is very unlikely
to change the basic story.
The other big unknown is of course
what we humans will do. Will we carry on burning fossil fuels
regardless, or will we actually succeed in kicking the habit and switch
to carbon-free sources of energy? But that’s a societal question – not a
scientific one.

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