A banking analogy for climate


in Daily updates, Economics, Science, The environment

[Update: 22 January 2009] Some of the information in the post below is inaccurate. Namely, it implies that some level of continuous emissions is compatible with climate stabilization. In fact, stabilizing climate required humanity to have zero net emissions in the long term. For more about this, see this post.

Every day, new announcements are made about possible emission pathways (X% reduction below year A levels by year B, and so forth). A reasonable number of people, however, seem to be confused about the relationship between emissions, greenhouse gas concentrations, and climatic change. While describing the whole system would require a huge amount of writing, there is a metaphor that seems to help clarify things a bit.

Earth’s carbon bank account

Imagine the atmosphere is a bank account, denominated in megatonnes (Mt) of carbon dioxide equivalent. I realize things are already a bit tricky, but bear with me. A megatonne is just a million tonnes, or a billion kilograms. Carbon dioxide equivalent is a way of recognizing that gasses produce different degrees of warming (by affecting how much energy from the sun is radiated by the Earth back into space). You can think of this as being like different currencies. Methane produces more warming, so it is like British Pounds compared to American dollars. CO2 equivalent is basically akin to expressing the values in the ‘currencies’ of different gasses in the form of the most important one, CO2.

Clearly, this is a bank account where more is not always better. With no greenhouse gasses (GHGs), the Earth would be far too cold to support life. Too many and all the ice melts, the forests burn, and things change profoundly. The present configuration of life on Earth depends upon the absence of radical changes in things like temperature, precipitation, air and water currents, and other climatic factors.

Assuming we want to keep the balance of the account more or less where it has been for the history of human civilization, we need to bring deposits into the account in line with withdrawals. Withdrawals occur when natural systems remove GHGs from the atmosphere. For instance, growing forests convert CO2 to wood, while single celled sea creatures turn it into pellets that sink to the bottom of the ocean. One estimate for the total amount of carbon absorbed each year by natural systems is 5,000 Mt. This is the figure cited in the Stern Review. For comparison’s sake, Canadian emissions are about 750 Mt.

Biology and physics therefore ‘set the budget’ for us. If we want a stable bank balance, all of humanity can collectively deposit 5,000 Mt a year. This implies very deep cuts. How those are split up is an important ethical, political, and economic concern. Right now, Canada represents about 2% of global emissions. If we imagine a world that has reached stabilization, one possible allotment for Canada is 2%. That is much higher than a per-capita division would produce, but it would still require us to cut our present emissions by 83%. If we only got our per-capita share (based on present Canadian and world populations), our allotment would be 24.5 Mt, about 3.2% of what we currently emit. Based on estimated Canadian and world populations in 2100, our share would be 15 Mt, or about 2% of present emissions.

Note: cutting emissions to these levels only achieves stabilization. The balance in the bank no longer changes year to year. What that balance is depends upon what happened in the years between the initial divergence between deposits and withdrawals and the time when that balance is restored. If we spend 100 years making big deposits, we are going to have a very hefty balance by the time that balance has stabilized.

Maintaining a balance similar to the one that has existed throughout the rise of human civilization seems prudent. Shifting to a balance far in excess carries with it considerable risks of massive global change, on the scale of ice ages and ice-free periods of baking heat.

On variable withdrawals

Remember the 5,000 Mt figure? That is based on the level of biological GHG withdrawal activity going on now. It is quite possible that climate change will alter the figure. For example, more CO2 in the air could make plants grow faster, increasing the amount withdrawn from the atmosphere each year. In the alternative, it is possible that a hotter world would make forests dry out, grow more slowly, and burn more. However the global rate of withdrawal changed, our rate of deposit would have to change, as well, to maintain a stable atmospheric balance.

Here’s the nightmare possibility: instead of absorbing carbon, a world full of burning forests and melting permafrost starts to release it. Now, even cutting our emissions to zero will not stop the global atmospheric balance from rising. It would be akin to being in a speeding car with no control of the steering, acceleration, or brakes. We would just carry on forward until whatever terrain in front of us stopped the motion. This could lead to a planetary equilibrium dramatically unlike anything human beings have ever inhabited. There is a reasonable chance that such runaway climate change would make civilization based on mass agriculture impossible.

An important caveat

In the above discussion, greenhouse gasses were the focus. They are actually only indirectly involved in changes in global temperature. What is really critical is the planetary energy balance. This is, quite simply, the difference between the amount of energy that the Earth absorbs (almost exclusively from the sun) and the amount the Earth emits back into space.

Greenhouse gasses alter this balance because they stop some of the radiation that hits the Earth from reflecting back into space. The more of them around, the less energy the Earth radiates, and the hotter it becomes.

They are not, however, the only factor. Other important aspects include surface albedo, which is basically a measure of how shiny the planet is. Big bright ice-fields reflect lots of energy back into space; water and dark stone reflect much less. When ice melts, as it does in response to rising global temperatures, this induces further warming. This is one example of a climatic feedback, as are the vegetation dynamics mentioned previously.

In the long run, factors other than greenhouse gasses that affect the energy balance certainly need to be considered. In the near term, as well demonstrated in the various reports of the IPCC, it is changes in atmospheric concentration that are the primary factor driving changes in the energy balance. Things that alter the Earth’s energy balance are said to have a radiative forcing effect. (See page 4 of the Summary or Policy Makers of the 4th Working Group I report of the IPCC.)

What does it mean?

To get a stable atmospheric balance, we need to cut emissions (deposits) until they match withdrawals (what the planet absorbs). To keep our balance from getting much higher than it has ever been before, we need to do this relatively quickly, and on the basis of a coordinated global effort.

{ 15 comments… read them below or add one }

Litty September 28, 2007 at 5:32 pm


To extend the analogy: right now we are doing donuts on the lawn, with the accelerator pushed firmly against the floor.

Sir Nerdington September 28, 2007 at 11:35 pm

This Onion article is amusing.

I like how the diagram included is clearly of ITER, which is actually being built in France.

Milan September 29, 2007 at 4:48 pm

Unrelated to anything: this photo of a Delta II rocket launch is very cool.

. October 4, 2007 at 4:09 pm

“There is unanimous agreement among the coupled climatecarbon
cycle models driven by emission scenarios run so far
that future climate change would reduce the effi ciency of the
Earth system (land and ocean) to absorb anthropogenic CO2.
As a result, an increasingly large fraction of anthropogenic CO2
would stay airborne in the atmosphere under a warmer climate.”

IPCC WG1 Report, Chapter 10, Page 4

Milan October 10, 2007 at 10:06 pm

∑ 5 Gt CO2e

Anonymous October 12, 2007 at 9:57 am

“In January 2007, the European Commission issued a communication stating that “the European Union’s objective is to limit global average temperature increase to less than 2°C compared to pre-industrial levels”.


Weaver’s team used a computer model to determine how much emissions must be limited in order to avoid exceeding a 2°C increase. The model is an established tool for analysing future climate change and was used in studies cited in the IPCC’s reports on climate change.


A 100% reduction of emissions saw temperature change stabilise at 1.5°C above the pre-industrial figure. With a 90% reduction by 2050, Weaver’s model predicted that temperature change will eventually exceed 2°C compared to pre-industrial temperatures but then plateau…
“People are easily misled into thinking that 50% by 2050 is all we have to do when in fact have to continue reducing emissions afterwards, all the way down to zero,” Lenton says.”


Milan October 17, 2007 at 2:47 pm

Climate equity: Andrew Pendleton
On how to divvy up responsibility for climate change

“This is the climate equity challenge, and it must now be set in the context of largely non-negotiable global emissions reductions on the order of 80 percent by 2050. As an organisation that advocates on behalf of the world’s poorest communities, Christian Aid supports the least risky and therefore most stringent pathway to decarbonisation. Therefore, step one in defining climate equity is to set a global carbon budget, within which all countries, industrialised and developing, have a legal obligation to remain.”

Pendleton leads the climate change policy work at Christian Aid.

. October 17, 2007 at 5:13 pm

Michael Meacher, the former UK Environment Secretary, sets a stringent timeline:

“Moreover, even this seemingly unreachable 60% cut by 2050 is still nowhere near enough. The
latest science indicates that a cut of no less than 90% is necessary by the much earlier date
of 2030 if we are to keep carbon concentrations in the atmosphere below 430 parts per million.
The significance of this threshold is that above this level we may not be able to prevent some of
the potentially catastrophic feedback processes from kicking in: such as the dieback of the
Amazon rainforest, the release of billions of tons of methane hydrates from the ocean floor, or the
collapse of the Greenland and Antarctica ice sheets. After that, nature takes over and the
biosphere becomes the primary producer of carbon. The global warming process takes on a
momentum of its own, beyond our control” (Meacher 2007).

Many of the works surveyed in this report express this urgent view, and argue that atmospheric
greenhouse gases should be reduced from the current level of 430ppm to 350-400pmm CO2e
(Meinshausen 2005, Retallack 2005, Harvey 2006a), requiring further emission cuts to near zero.
Britain’s Environment minister David Miliband says “essentially, by 2050 we need all activities
outside agriculture to be near zero carbon emitting if we are to stop carbon dioxide levels in the
atmosphere growing” (Miliband 2006).

Anon October 19, 2007 at 1:38 pm

When the science tells us that, to have a high probability of holding total warming (since pre-industrial) to 2C degrees — a widely endorsed maximum, but by no means a “safe” one — global emissions must peak somewhere around 2015? When the more we overshoot 2C the faster we’ll need to pull down post-peak emissions, if, that is, we want to keep the warming to “manageable” levels? When, to quote John Holdren’s bitterly precise summary, “We already know the future, and it’s some combination of mitigation, adaptation, and suffering”?

Anonymous November 1, 2007 at 10:24 am

Wildfires turning northern forests into carbon-dioxide sources

A group of U.S. researchers have found that wildfires — fuelled by climate change — may be turning boreal forests into sources of carbon dioxide.

The boreal forests — found in northern Canada, Alaska, Siberia, China, Scandinavia and elsewhere — make up the second largest type of forest in the world behind the tropical rainforest.

Scientists have historically believed that the boreal forests act as a carbon sink, as trees absorb carbon emissions and reduce them in the atmosphere.

But new research from the University of Wisconsin-Madison, published in the Nov. 1 issue of the journal Nature, has found that the forests may be emitting more carbon than they are absorbing.

. November 14, 2007 at 9:53 am

Report: Carbon Removal Has Little Impact

According to the new U.S. study, North America released 1,856 million metric tons of carbon into the air in 2003 — 85 percent from the United States, 9 percent from Canada and 6 percent from Mexico.

At the same time, growing vegetation and other sources took in about 500 million metric tons of carbon.

This is the first net carbon report for the region, said Tony King, lead researcher on the report and chief scientist at Oak Ridge National Laboratory.

. November 15, 2007 at 11:35 am

Evidence is fast mounting that time is running out for nations to unite in a credible response to climate change. The International Energy Agency said last week that energy-related emissions of carbon dioxide are set to grow from 27 gigatonnes in 2005 to 42 gigatonnes by 2030 — a rise of 56%. Other estimates project even higher growth, and also reveal, alarmingly, that ‘carbon intensity’ — the level of carbon emissions required to sustain a given level of economic activity — is actually growing again.


. February 29, 2008 at 11:33 am

‘Stabilizing climate requires near-zero emissions’
A new climate science paper calls for dramatic action



. June 9, 2008 at 11:04 am

The fraction of CO2 remaining in the air, after emission by fossil fuel burning, declines rapidly at first, but 1/3 remains in the air after a century and 1/5 after a millennium (Atmos. Chem. Phys. 7, 2287-2312, 2007).

Milan August 13, 2008 at 12:10 am

This discussion is quite relevant to the above.

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