Download Nissen AA

2015-02-02
Arctic Amplification: Science and Engineering
Arctic amplification (AA) is can be defined in several ways and is therefore a rather
confusing term. The commonest definition is the ratio of Arctic warming to global warming.
However this is very imprecise.
If we define it as the ratio of the rate of Arctic surface temperature rise to the rate of surface
temperature rise at lower latitudes, then it is a variable. The rates have to be averaged over a
period, e.g. a decade, in order to get a meaningful value. If the average over the last 16 years
is taken, then the value of AA comes out at a high value because there has been so much
warming in the Arctic and so little at lower latitudes. In a recent paper it is estimated at eight
times as much. This would be commensurate with polar warming of 1 degree C while the
warming at lower latitudes was 0.125 degrees C over the same period.
But many scientists argue that global warming has continued over the past 16 years – it is just
that most of the heat has gone into the oceans. They are thus treating “warming” as the extra
heat resulting from an imbalance of radiative forcing – typically estimated in the region of 1.6
to 1.8 W/m2 (Watts per square metre). They will often say that AA has remained at around
two. This implies a definition of AA that compares extra heat in the Arctic compared to the
extra heat globally. They then take the temperature rise over a long period, typically 50
years, to say that the extra heat in the Arctic must be about double that at lower latitudes to
explain the greater rise in temperature of the Arctic over that period. Thus AA is not a
dynamic entity, changing from decade to decade, but a relative constant parameter.
Unfortunately this common treatment of AA does not reflect the accelerated warming in the
Arctic and the associated non-linear effects this is having, particularly on the behaviour of the
jet stream and hence weather extremes in the Northern Hemisphere.
Furthermore, this common treatment of AA helps to obscure what is happening in the
dynamics of the Arctic. And it reinforces the idea that “amplification” is just a multiplier of
temperature, which happens naturally because of some “amplification mechanism” which is
not well understood. The belief is that there is something about the Arctic which leads
naturally to this amplification. This is taking a naïve view that amplification is similar to an
electronic amplifier which takes the input of an electrical signal and produces an output
which is linearly related to the input. So, for an AA factor of two, the Arctic is taking in a
temperature or a warming signal as an input and producing twice the output.
One suggested mechanism is that the Arctic is twice as sensitive to greenhouse gases as at
lower latitudes. This is never explained – at least I have never seen a convincing explanation
for why this should be so.
Another suggested mechanism is positive feedback. As the sea ice retreats, more heat is
absorbed into the ocean which causes further retreat. But this is where scientists betray a lack
of understanding of the nature of positive feedback. Positive feedback produces a non-linear
effect over time: an acceleration of the output as it feeds back to the input. In an audio
system, the sound from the loudspeaker is fed back into a microphone, one gets a howling as
the amplifier reaches its limit. Only by providing negative feedback with the positive
feedback can a linear amplification of the input signal be produced.
Thus if there is positive feedback in the Arctic, it should produce a non-linear effect. In fact,
this is what is observed, with the volume following an exponential downward trend. There is
effectively a vicious cycle of warming and melting at play. AA needs to be treated as a
variable, whose value has been increasing.
While the Arctic temperature has been continuously rising, the temperature at lower latitudes
has stayed fairly constant since the last big El Niño in 1997/8. This gives an anomalously
high value to the ratio.
Rather than look at AA, we need to consider what has been happening to the temperature
gradient between the Arctic and lower latitudes. This gradient has reduced markedly over the
past 16 years, since that El Niño. And this has been reflected in the behaviour of the jet
stream and the number and duration of extreme weather events. Thus the extreme hot, dry,
cold and wet spells have been more frequent and have lasted longer, so there have been worse
droughts, floods and blizzards.
The heat and precipitation levels have risen as a result of global warming to make some of
the extreme weather that much worse. And global warming has accentuated the temperature
difference between land and ocean, to make the jet stream stick more easily. But it is the
Arctic warming which has been driving the changes in Northern Hemisphere climate, to the
extent that one can attribute the observed abrupt climate change to the reduced temperature
gradient between the Arctic and lower latitudes.
The implications of this are enlightening. If the Arctic continues to warm, the abrupt climate
change will get worse. And if the Arctic warming continues to accelerate, this change in
climate will accelerate. On the other hand, if Arctic could be halted, then the non-linearity in
climate change would be removed. And if the Arctic temperature could actually be reduced,
then there could, at least to some extent, be a restoration of jet stream behaviour and a
reversal of the abrupt climate change which the altered behaviour had produced.
The people who claim that reducing our carbon emissions can save the Arctic are talking
nonsense. Those who use the warming of the Arctic as a reason for reducing our carbon
emissions are being misleading and are downplaying the importance of action to cool the
Arctic.
Now, let us consider how the positive feedback got going. Has it happened before?
The Milankovitch cycles have driven a warming of Northern Hemisphere summers since the
last glacial maximum (LGM) about 20,000 years ago. There are indications that the positive
feedback did produce a removal of the ice cover of the Arctic Ocean for extensive periods,
during which meltwater pulses raised the sea level at rates of around 5 cm per year. The
heating in the Arctic to produce these pulses could have come from the loss of albedo, with
the low amount of sea ice in summer.
But the Arctic Ocean has been covered in ice for the past 8000 years. There has been no
sudden large change in sea level since the “8000 year event” which raised the sea level many
metres. During that event, the ocean might have been flooded with fresh water which stayed
on the surface and allowed sea ice to form.
Since then, there must have been some negative feedback, to stabilise the extent of the sea
ice. There are three candidates for this: meltwater diverting the Gulf Stream, the Atlantic
Meridional Overturning Circulation (AMOC) and thermal radiation from exposed ocean.
Meltwater from Canada and West Greenland produces a cold fresh surface layer in the NW
Atlantic and tends to push the Gulf Stream and associated transatlantic currents further south.
This reduces the amount of the warm water entering the Arctic.
The effect of the AMOC is not well understood. When the Arctic warms, there is a reduced
quantity of cold saline water sinking to the bottom of the ocean as ice forms. The AMOC is
thus weakened. This would be a positive feedback, except that the temperature of the sinking
water is increased which is a negative feedback. It is not clear whether the net effect is
positive or negative feedback.
There is definite evidence that thermal radiation can provide negative feedback. As the sea
ice retreats, there is more open water. This not only absorbs incoming solar radiation, heating
the ocean at depth in a positive feedback, but it produces outgoing thermal radiation, cooling
the ocean surface and encouraging the formation of ice in a negative feedback. In the year
after a significant retreat there tends to be a rebound, suggesting that the negative feedback
dominates.
Thus it is a reasonable hypothesis that there has been a net negative feedback to keep the sea
ice at a fairly constant level over the 8000 years until a few decades ago.
But then, within the past thirty years, two new effects have come into play: the Gulf Stream
and currents across the Atlantic have been warming faster as the SO2 level reduced; and
greenhouse gases have further increased. Significant warming of water entering the Arctic is
probably the main reason for the start of sea ice retreat in the 80s – the temperature of the
water may have been higher than at any time in the past 8000 years. However greenhouse
gases would also have reduced the negative feedback for outgoing thermal radiation. These
two effects assured that the positive albedo feedback dominated as the sea ice started to
decline, producing the observed exponential decline of sea ice over the past thirty years.
So far I have given a qualitative analysis based on looking at the components. One also
needs to look at how these components fit into an Earth System has been designed (or selfselected) to produce the required effects for human evolution and development of civilisation.
On the one hand, the Earth System had to produce wildly fluctuating climate in order for the
evolution of a species capable of adapting to different conditions and working socially
towards solutions to a wide range of problems. On the other hand, the Earth System had to
produce stable climate, sea level and temperature to give optimum conditions for the
development of civilisation over some thousands of years.
We are looking for the Earth System solution to what is essentially an engineering problem.
The Milankovitch cycles provided a mechanism for obtaining wild fluctuations between
defined limits; but there had to be a means of suddenly producing a period of extraordinary
stability with constant climate (for reasons of agriculture) and with constant sea level and
dominant wind patterns (for reasons of harbours and sea trade with sailing ships).
We now have to do a certain amount of speculation. It was important to have a rapid increase
in sea level after the LGM in order to make it easier to have a subsequent period of constant
sea level. The temperature also needed to become nearly constant in order to have the sea
level constant, so this period had to be close to the maximum of the Milankovitch warming.
There had to be a means of fine-tuning of temperature – and it turns out that human activity
was able to provide this, almost as a Gaia-like feedback. The Milankovitch cycles had started
into a cooling part of the cycle, but the cooling was exactly countered by CO2 and methane
greenhouse gas warming. When it got a little too hot, human population was reduced and the
greenhouse gas warming declined – in a kind of negative feedback. Conversely, when it got
a little too cold, more burning took place and greenhouse gas warming increased.
One can now see that the rapid sea level rise would be produced by a heat generator, which
could be switched on and off. It would be switched on by rising temperature in the Northern
Hemisphere resulting from Milankovitch cycles. It would need to be switched off just prior
to the period of stability.
Now one can see how it might have worked. The heat generator would be the Arctic Ocean
in a stable low ice state. It could be switched into this state from a stable ice covered state by
warm water streaming in from the Atlantic. The function of the so-called 8000 year event
was to switch the Arctic Ocean out of the low ice state back into the ice covered state. This
event was produced by the overflowing of a huge ice-dammed lake. The cold fresh water
would have flooded into the Arctic, forming a low salinity surface layer and allowing the sea
ice to become thick during autumn and winter, while insulating the surface layer from
warmer saline water below.
If this theory is correct, then it explains why extra warming of the Gulf Stream and
transatlantic currents (as a result of anthropogenic greenhouse gas emissions) has started
switching the Arctic Ocean from its ice covered state into the low ice state. In fact this
transition could be nearly complete. After the record retreat of sea ice in 2012, it only
requires one further major retreat of sea ice to bring it down to a level when conditions could
change such as to lock the Arctic Ocean into the low ice state.
The current condition of the sea ice is conducive to a major decline next September. Much of
the multi-year ice has exited through the Fram Strait. The sea ice concentration over much
of the Arctic Ocean is less than 100%, implying that the ice is broken. Wave action can
easily further break up the sea ice far from open water, since viscosity is reduced. A major
storm in July or August would mix the upper layers of the ocean bringing warm saline water
to the surface – water which has been heated through lack of sea ice cover.
Let us consider the probability distribution for a ‘blue ocean’ event, marking entry of the
ocean into the low ice state – effectively a point of no return.
There are two ways of looking at this: one trend oriented and one random oriented. If we
consider September 2014 “recovery” as a statistical outlier, the exponential trend of sea ice
volume is towards a zero somewhere between 2016 and 2017. There is thus approximately
50% chance of the blue ocean event in September 2015 and 2016. The chance of it
happening in 2016 might be slightly greater than in 2015 according to the trend. This might
give say 20% chance for the event in 2015 and 30% in 2016, but unlikely to give as little as
10% in 2015 with 40% in 2016 or as much as 30% in 2015 with 20% in 2016.
The other way is to consider the random chance of conditions for the blue ocean event “in
one go” (because the sea ice level is already so low). The highest probability will be for next
September. Because of the randomness, there will be an exponential decay. The rate of
decay can reflect the trend, such that perhaps there’s a 25% chance of sea ice lasting more
than four years. (Peter Wadhams gives about 20% chance of sea ice lasting more than five
years.) If we have a halving in probability every two years, we obtain a probability sequence
approximately as follows: 29%, 21%, 14.5%, 10.5%, 7.25%, 5.25%, etc. Each pair adds up
to half the previous pair, the first four years adding to 75% and the total adding up to 100%.
The distribution might start higher with a steeper decline or lower with a less steep decline.
But the probability for the first year is likely to lie somewhere in the range 20 to 40%.
The ‘best’ distribution is going to be somewhere between these two ‘solutions’. Thus we can
estimate the probability of the blue ocean event in 2015 as between 10% and 40% with
moderate confidence.
I hope this gives you a sense of the urgency for cooling the Arctic to stave off the blue ocean
event – an event marking permanent entry into the low ice state, with disastrous
repercussions for all humanity.