Fingerprinting a Climate

Climate simulation models are usually 250 years in scope – 150 years into the past and 100 years into the future.  The models have time intervals of 20 minutes.  Many millions of calculations have to be undertaken for every 20 minute time-step, so huge super-computers are required to do the job.  Even with such super-computers, as at the UK’s Met Office, it still takes 3 months to run a complete simulation.

To test a climate model, a simulation is undertaken of the observed 0.8 degree Celsius warming of the globe over the past 150 years – a process called fingerprinting.  In this way, the simulation’s predictions can be tested against observed data.  The models are built using the factors that climate scientists think have affected climate in that time:  natural factors such as volcanoes and variations in the output of the sun; and human factors, most particularly the increase of carbon dioxide resulting from burning fossil fuels and deforestation. 

When the models are run using only only natural factors, they can reproduce the observed climate data, but only until about 1970.  After 1970 the models and the observed data diverge, and in fact the models tend to show a period of planetary cooling.  When the human factors, particularly carbon dioxide emissions, are included in the models, then the real, observed global warming and that predicted by the simulation, coincide.

This ability to pull out the effects of natural and human factors not only tests the models, but also allows scientists to accurately attribute the effects that human activity is having on the Earth’s climate.  This process of fingerprinting therefore allows the Intergovernmental Panel on Climate Change to make such definitive statements about the effect we are having on our climate.  Their fourth report states that “There is at least a 90% chance that the observed increase in temperature globally is due to man-made greenhouse gases”. 

The challenge then is to work out how to project climate change into the future, which is a far more challenging process as there are so many variables, including:

  • How will the land and sea sinks behave when concentrations of atmospheric carbon dioxide increase?
  • How will human population grow?
  • How will human behaviour change, regarding levels of carbon dioxide output?

To model future climate change, scientists create a number of different scenarios for these unknown factors.  These seem to result in a spread of possible future global warming over the next 100 years from 2-6 C.  The current worst case of 6C constitutes a warming of more than that between the last ice age and now, but 100 times faster.  And this has the ability to tip us over the edge into a series of horrific scenarios, such as the slowing of the Gulf Stream, and the melting of the Antarctic and Greenland ice sheets, the latter resulting in a sea level rise of more than 10m. 

Decoding the Keeling Curve from Mauna Loa

Since 1958, Charles David Keeling, then his son, Ralph Keeling, have made a continuous recording of carbon dioxide levels in the atmosphere at the Mauna Loa Observatory in Hawaii.  This has produced the now iconic Keeling Curve, showing a steady increase in concentrations of levels of atmospheric carbon dioxide from the 1950s to the present day.  This increase roughly correlates with increases in burning of fossil fuels, and also with levels of deforestation.  Currently 90% atmospheric carbon dioxide increase comes from fossil fuels and 10% from deforestation.

Key to the history of the graph is a tale of individual determination in the face of funding cuts and general disinterest at the start of the CO2 recording process.  Keeling had also been recording CO2 levels at the North Pole, but these recordings were abandoned when funding was cut.  In the face of these cuts, Keeling kept going with the Mauna Loa record, despite having few resources.

The graph is not a steady line.  It curves upwards with regularly spaced wiggles and with irregularly flattened sections on the curve. Scientists have spent much time decoding these readings, and are continuing to do so.

For example, it has been found that the regularly spaced wiggles on the graph relate to the annual growth rate of vegetation in the northern hemisphere.  In the summer the vegetation sucks down CO2 and releases it in the winter when it decomposes.  This results in a regular wiggle on the curve.

There are also irregularly spaced flat areas on the curve, showing variation in the year to year growth rate of CO2.  These can sometimes be tied to specific global events, such as a decrease in the level of CO2 after the PInatubo eruption.  In the main, however, this annual variation in growth rate has been found to relate to how tropical lands, particularly tropical forests, are responding to anomalies in tropical weather and temperature.  Spikes in the annual growth rate of carbon dioxide in 2005 and 2010 can be seen to relate to drought in Amazonia.  And plotting the annual growth rate of CO2 against tropical temperature anomalies produces a very tight correlation, reflecting how tropical temperature affects how much carbon is absorbed by tropical forests.

This week, scientists from Peking University published a paper in Nature (Wang et al, 2014), analysing how flat spots in the annual mean line have changed sensitivity through time in the period 1970 to 2013.  In the 1970s, spikes in tropical temperature were producing lower spikes in CO2 than they do today – in 2013, temperature spikes caused a doubling of CO2 levels compared to the rate in the early 1970s.  This paper does not propose an explanation as to why this is happening.  Professor Peter Cox suggests a couple of potential reasons, such as a long term trend of drying in the Amazon, or do with variation in the ocean?

The Mauna Loa record is a key climate change record, which scientists are working hard to decode, comparing it with other data sets to reveal new questions, and sometimes new answers.  We need to be very grateful to the tenacity of one family, the Keelings, who worked so hard to keep the record going.  Without it, our understanding of how we are affecting the carbon cycle, and how in turn that is affecting our lives, would be so much less.

Every time look at Mauna Loa record and compare with other data sets, finding new questions, and sometimes new answers.

A tale of two poles

Today I found out a number of interesting things about the Earth’s poles and their ice:

1) The Antarctic is colder than the Arctic.  Since ice has not melted there yet, it is not currently contributing to sea level rise.

2) The melting of sea ice (as with much of the Arctic) does not contribute to sea level rise, as the floating ice already displaces the volume of water it would occupy if it melted.  Therefore only the melting of land ice can raise sea levels (small glaciers and Greenland ice sheet).

3) In our warming world, temperature rises have been most marked in the polar Arctic regions.  “Since 1980, the surface area of the Arctic sea ice in summer has formed from around four million square kilometres to about one and a half million square kilometres. The Arctic is a microcosm of climate change. It’s a place where changes in the Earth’s temperature have been felt most keenly.” (Mat Collins, University of Exeter).

Another key difference is that in September 2012:

  • The Arctic had the smallest area of sea ice ever recorded by satellite
  • The Antarctic had the largest area of sea ice ever recorded by satellite


400 parts per million

On Thursday May 10th 2013 the Mauna Lao Observatory in Hawaii recorded the concentration of carbon dioxide in the Earth’s atmosphere passing the 400 parts per million mark for the first time since records began in 1958.  What the records (known as the Keeling Curve after the scientist who devoted his life to collecting them) show is that concentrations of carbon dioxide in the Earth’s atmosphere have been climbing steadily in that time.

Ice core data, obtained from analysis of the gases trapped bubbles in polar ice, go back 800,000 years, and in this time the planet has not experienced this level of carbon dioxide.  Analysis of carbon isotopes present in compounds made by tiny marine phytoplankton preserved in ancient ocean sediments suggests that the last time the planet experienced 400 parts per million was in the Pliocene, between 5.3 and 2.6 million years ago.

In the mid-Pliocene warm period, 3.3 to 3 million years ago, the Earth had several other key similarities with the contemporary Earth – a similar intensity of sunlight reaching the Earth; similar global geography; similar parameters of the Earth’s orbit; and the fact that many mid-Pliocene species are still extant.

There are also some key differences between these two Earth phases.  Sea levels were higher (estimates range from 5 to 40 meters higher than today); the Greenland and Antarctic ice sheets were smaller than today; the jet stream was sluggish, if it existed at all – the planet existed in a permanent El Nino.  Average temperatures were 3 or 4 C higher than today, and possibly 10 C higher at the poles.

For these key reasons, many scientists consider the mid-Pliocene to be a key climate analog for the Earth’s future.  However, there is one key reason that makes this analogy so particularly significant for climate science:  in the mid-Pliocene, the levels of carbon dioxide in the atmosphere were decreasing. Today they are increasing at an unprecedented rate.  During ancient climate change, an increase of 10 parts per million might have taken 1,000 years or more to happen. Now, if emissions continue to increase as predicted, the planet is set to reach the 1,000 ppm level in only 100 years.

How do volcanoes affect climate change?

There are a number of natural forcing factors at work on the earth’s climate, causing short-term changes to weather patterns and longer-term climate change.  It is against this backdrop of natural variability that the effects of human-induced climate change must be measured.   Of these, volcanoes are a key forcing mechanism, which have had large impacts on human civilization in the recent past.  For example, in northern Europe, the medieval Little Ice Age corresponded with a period of high volcanic activity, whilst the Medieval Warm Period occurred at a time of low volcanic activity.  Solar variation was also a key factor in these climate changes.

Volcanic forcing can have both short-lived and longer-term impacts on global climate.


  • Sulphuric aerosols injected up into the stratosphere reflect back nearly all the sun’s energy that they encounter, sending it back into space and preventing it from reaching the planet’s surface.  This is a short-lived effect that can be measured in terms of years.
  • Volcanic ash, a form of black carbon, can be deposited on snow fields, glaciers and other reflective bodies, reducing their albedo.  Hence they reflect less of the sun’s energy and absorb more of it.  This would result in a short-term warming, until the deposits are covered by further snowfall.


  • The volcanoes inject carbon dioxide and other long-lasting blanket gases into the atmosphere, and their effect can be measured in centuries.  This have a warming effect on global climate.

The effect of volcanoes on climate in the short and long term has been used by some climate change sceptics to attempt to undermine the credibility of climate science.  However, as the British Geological Survey puts it:

“The contribution to the present day atmospheric CO2 loading from volcanic emissions is … relatively insignificant.”

…especially when compared with the effects of anthropogenic carbon dioxide pollution.  Volcanoes contribute c.100–300 million tonnes of CO2 each year; which is only 1% of the annual anthropogenic carbon dioxide load.

The carbon cycle – warm blankets and snowballs

During the earth’s very early history, around 4 billion years ago, the sun radiated 25 – 30% less heat than it does today.  If the earth had the same blanket of gases as it does today, the earth should have been c.20C colder.  However, there is evidence (the presence of water and the development of life, 3.8 billion years ago) that the earth was not this cold, and was in fact warmer than today.  The reason for this lies in the carbon cycle.

The early earth was apparently covered by a warm blanket of carbon dioxide gas, acting as a blanket, trapping a proportion of the heat of the sun and re-radiating it back to the surface of the earth.  This carbon dioxide was itself part of a carbon cycle: carbon dioxide was dissolved in rainwater to become carbonic acid; this rained down to the earth’s surface; it weathered silicate rocks to make bicarbonate ions; which in turn entered the sea water as bioavailable calcium taken up by shellfish to make their shells; these in turn were deposited at the bottom of the seas as limestone.  This carbon cycle was triggered by the enlargement of the continents, making more material available to be weathered by the carbonic acid and thus more carbon to be taken out of the atmosphere and deposited in the earth’s crust.

As the continents continued to expand, the reduction of carbon dioxide in the atmosphere caused the planet to cool and more polar ice to be created, in a negative feedback loop.  As the continents continued to expand, so the planet contiuned to cool, and the ice expanded further away from the poles.  When the ice reached the tropics, the planet entered into a tipping point, and the ice closed over the entire planet, meeting at the equator.  This has happened twice in the earth’s history, at 2.2 billion and 700 million years ago, during periods known as ‘snowball earth’. 

Again the carbon cycle played a key role here, this time as a positive feedback loop, reversing the earth’s snowball.  Volcanoes continued to spew carbon dioxide into the atmosphere, but the time the land was so covered by ice that the carbonic acid could not weather the rock and be deposited in the earth’s crust.  The carbon dioxide thus built up in the atmosphere, developing as a warming blanket that eventually absorbed and re-radiated enough energy back to the earth’s surface to start to melt the ice and reverse the snowball. 

I find it fascinating that the lime products I am using to renovate my house are in fact carbon stores from billions of years ago, in the same way that coal is a carbon store.  However, there is a key difference between these two materials.  When coal is burnt to produce heat, the carbon inside it is released as carbon dioxide which serves to warm the planet as a blanket gas.  When limestone is burnt to produce lime, carbon dioxide is also produced, however, the lime reabsorbs this carbon when it is used, and allowed to harden. 

Reflections at the end of week one

The big surprise for me this week was realising how little I know about the earth systems mechanics of climate change.  I have long been concerned by climate change, beginning as a direct activist against tropical logging, new roads and other climate damaging developments in the early 1990s. This activist experience, combined with many conversations, books read, internet research and 25-year old GCSE chemistry, had led me to be fairly confident that I knew what I was talking about when discussing human-made climate change.  Now I find I was wrong, that the greenhouse effect does not operate the way I thought and that carbon dioxide is not the main climate altering gas.

I am delighted to have my assumptions challenged, and my knowledge realigned.  In this way I can become a more effective communicator and campaigner.  Which I still need to be. Because, as my peer group and internet research is continuing to tell me, while I may have been wrong about the earth science, I am not wrong about the devastating potential of human-induced climate change.  I may not have understood the mechanics, but I understand the effects all too well.