Understanding Time Lags Important for Climate Policies.
“Cumulative
carbon”, the fact that human emissions of CO2 are removed only slowly
from the atmosphere through the natural carbon cycle, is the essence of the
climate change problem. Given the thermal inertia of our planet it may produce
a substantial time lag between effective action, to limit emissions, and actual
stabilisation of temperatures, equivalent to the operation of a domestic
heating radiator on a one way ratchet. Some climate scientists believe this effect
may have been exaggerated, and will be largely offset by other elements in the natural
cycle. Even so there is a consensus that we need to move to a world of net zero
emissions and beyond.
So much of the predictive element of climate science has
been borne out by observation that it is easy to forget there are still major
gaps in our understanding and major uncertainties that are very relevant to
understanding what climate policies are necessary to (ultimately) stabilise
global temperatures.
The slow but seemingly relentless upward trend in global surface
temperatures sits firmly in the middle of past model predictions, and denials
that it is actually happening (the famous Lawson “hiatus” based on cherry
picking outlier el Nino effects), or that it has nothing to do with human
contributions to greenhouse gas concentrations, look increasingly threadbare
and ridiculous. There is no doubt that human-induced climate change is with us,
and that it is dangerous.
However, although the science makes it clear that urgent
emission reductions are essential, it is much less clear how much leeway we
have, and whether the 1.5o C or 2.0o C “targets” are
attainable. The uncertainties manifest themselves in discussions over the time
lags involved, for example between stabilisation of the atmospheric concentration
levels of CO2 and stabilisation of global temperature. These questions relate
in turn to complexities of the natural carbon cycle and the
The issue of time lags is in some ways politically important.
Thomas Stocker, then co-chairman of the IPCC
Working Group I (assessing scientific aspects of the climate system and climate
change), and addressing the Environmental Change Institute in Oxford in 2014,
argued that “committed peak warming rises 3 to 8 times faster than observed
warming”. The implication is that there are very substantial time lags. In this
case temperatures could continue to rise, perhaps for several decades or even
centuries, even if net human emissions were reduced to zero. Similar comments
can be found from other climate scientists. It has even been suggested that the
thermal inertia effect, considered on its own, could stretch the lag to about
200 years – the time for heat equilibrium adjustment to reach the deep oceans.
The intuitive physical explanation of long time lags is simply
the phenomenon of thermal inertia. When we turn up the radiator at home it may
take an hour or more for the room to reach a new equilibrium temperature. Global
warming, in this analogy, is a radiator heating system on an upward ratchet. The
issue for humanity is that by the time temperatures become really uncomfortable,
we have already ratcheted up the future temperature to which we have committed.
If this is a real danger it dramatically increases the risks associated with
climate inaction or “business as usual” trends. It is also creates an alarming
image of climate change as a kind of doomsday machine in which humanity is
trapped through its failure to anticipate and respond.
However rather more optimistic views have been presented by
other scientists. In 2014 Ricke and Caldeira[1] argued that the lags had
been seriously overstated, suggesting a time lag of only ten years as a more appropriate
estimate. Oxford-based climate scientists, such as Myles Allen, have also
suggested that stabilising atmospheric concentrations could lead to a comparatively
early stabilisation of global temperature.
The main reason is the potential offsetting effect of the absorption
of incremental carbon through the various elements of the natural carbon cycle,
including ocean CO2 uptake and the behaviour of biosphere carbon
sinks. However there is no natural law that requires such a balance of effects,
and the reality seems to be that we are trying to compare two magnitudes, both
of which are of major importance but are also difficult to measure with
precision. If the effects broadly cancel out over particular timescales, this is
essentially a numerical coincidence within the modelling effort. We can expect
future research, better measurement, and associated modelling, will gradually
improve our understanding.
But melting ice caps are a separate story!
The above discussion does not however cover all the long time
lags involved. One particular concern has to be the polar ice caps. If global
temperatures reach the point where these start to melt, then the consequential effects
in the form of rising sea levels will go on for centuries. Ice sheets, as
opposed to sea ice, can, like the retreating glaciers, only be restored by
precipitation. That will be slow and net annual ice gain will probably depend
on conditions associated with a fall in global and polar temperatures to or
beyond what used to be regarded as normal.
And the policy implications of these uncertainties?
In reality the consequences for policy of differing
estimates can be exaggerated, since there is agreement on most fundamentals.
Most modelling now recognises that stabilisation of temperature, even at a
higher level, requires progress to net zero human emissions. Importantly, this
is probably unachievable without substantial measures to remove CO2 (and other
gases) from the atmosphere. In practice processes for carbon sequestration are likely to be very expensive process. They represent a form of geo-engineering.
[1]Maximum warming
occurs about one decade after a carbon dioxide emission. 2014, Ricke and
Caldeira
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