Positive feedback could destabilise the climate

An introduction to the science of climate change


In this blog post, we hope to give you a quick introduction to the science of climate change. This is aimed at those who may not be familiar with the science behind the ideas, so some explanations have been simplified. We feel that understanding why climate change is happening is essential for understanding why we are campaigning for fossil fuel divestment, so please share this blog post with as many people as possible.

The ‘greenhouse effect’

The ‘greenhouse effect’ refers to the trapping of heat by certain gases known as greenhouse gases in the atmosphere [1]. This is entirely natural and it keeps the planet considerably warmer than it would be otherwise. Scientists estimate that without the greenhouse effect, global temperatures would be 33°C lower than they are today [2]. You can compare the effect to a car left in the sunshine: light passes through the glass, but the heat produced is unable to pass back out through the glass, causing the car to heat up.

The greenhouse effect

Fig. 1: The greenhouse effect (image source – US EPA 2012)

How can humans influence the climate?

People affect the climate by changing the amount of greenhouse gases in the atmosphere. An example of a greenhouse gas is carbon dioxide (CO2). CO2 is emitted into the atmosphere when we burn fossil fuels such as coal, oil and gas. Modern industrial society burns fossil fuels in vast quantities, providing electricity for your gadgets, aeroplane fuel for your next holiday flight, energy to manufacture fertiliser to grow our meals, and countless other uses. Apart from CO2, other greenhouse gases are also emitted by society. For example, methane (25 times stronger than CO2 in terms of contribution to the greenhouse effect [3]) is released in large quantities by animal farming; refrigerant gases (such as hydrofluorocarbons, which can be hundreds or thousands as times as strong as CO2 [3]) can leak from our fridges and freezers. Our emissions can change the amount of greenhouse gases in the atmosphere. This changes the strength of the greenhouse effect and the temperature of the planet.

What changes have been observed so far?

We have seen how it is theoretically possible for humans to affect the Earth’s climate – but have we actually seen evidence of any changes? In short, the answer is yes. Here are just a few examples of measured changes:

  • CO2 levels have increased by 40% compared to the pre-industrial era [4].
  • Surface air temperatures have increased by 0.85°C since 1880 [4].
  • Oceans have also warmed, taking up about 90% of the extra heat caused by warming [15].
  • Sea levels have risen by 10 to 20 cm over the last century [5].
  • Oceans have become more acidic due to dissolved CO2, causing the shells of some sea creatures to dissolve [6].

Feedback effects and tipping points

The Earth is a hugely complex system and does not react to changes in a simple way. Two types of feedback effect exist:

  • Positive feedback, or a vicious cycle, means that a little amount of warming triggers further warming.
  • Negative feedback, or a virtuous cycle, means that a little warming has an effect that slows down warming.

An example of a positive feedback loop is the release of methane from permafrost (permanently frozen soil, found near the poles) and the seabed. Huge amounts of methane are stored in permafrost and on the seabed in formations called ‘clathrates’. Recall that methane is a much stronger greenhouse gas than CO2. As permafrost melts due to higher temperatures, more methane is released into the atmosphere, causing further warming. Methane releases have been implicated in previous abrupt climate shifts [8]. Some reports suggest that methane is already being released from the Arctic [9].

Positive feedback could destabilise the climate

Fig. 2: Use of fossil fuels could destabilise the climate by starting a vicious cycle of warming

There are many more examples of positive feedback effects, e.g. warming oceans release more CO2; melting sea ice exposes dark sea which absorbs more heat; forests dry out and become a source of CO2, rather than absorbing it.

An example of a negative feedback effect is that as CO2 levels rise, trees and plants may absorb more CO2 from the atmosphere.

The question is, which has more influence – positive or negative feedback effects? Since the climate system is so complex, it is difficult to predict the exact outcome [7]. It has been suggested that warming of merely 1.5°C would be sufficient to thaw significant regions of permafrost and release large amounts of greenhouse gases into the atmosphere [12]. Note that today we are already at 0.8°C of warming, and are already committed to perhaps another 1°C of warming even if we stopped emitting greenhouse gases today, because the climate takes some time to respond to changes in the atmosphere [13].

The more fossil fuels we burn, the more we risk reaching a tipping point at which warming will speed up of its own accord and destabilise the climate irreversibly. We have to stop burning fossil fuels BEFORE we reach such a tipping point.

What does the future hold?

Here are just some of the predicted effects of climate change, as described by the Intergovernmental Panel on Climate Change (IPCC) [10].

  • Ecological
    • Global surface temperatures could rise by up to 4.8°C by 2100, depending on our levels of emissions. Bear in mind that the average temperature difference between the last ice age and today was only around 4 to 7°C (and this temperature rise took place over thousands of years), so what may sound like a small increase is actually very significant [14].
    • Many species face a greater risk of extinction since they cannot adapt fast enough to new climatic conditions.
    • Droughts and high temperatures may cause forests to die back, with associated loss of services that forests provide to us (e.g. clean water, wood production).
  • Human
    • Sea levels could rise by 0.3m – 0.8m by 2100 (depending on emissions), causing the flooding of low-lying countries and island states. Sea level rise may continue for centuries.
    • Agricultural yields will be reduced in many places and fisheries will be affected, meaning that providing food for everyone will be difficult.
    • Existing health risks would be made worse by climate change, e.g. diseases may spread to new geographical areas.
  • Economic
    • Climate changes impacts may slow down economic growth (this would happen before we actually run out of fossil fuels).
    • Poverty reduction will become more difficult.
    • Impacts will generally affect poor people and countries more than the rich.
    • There will be increased migration and conflict as climate change will worsen existing conflicts over resources.

It is worth noting that the models used in the IPCC’s latest report do not account for the effect of melting permafrost because they are difficult to predict accurately [11]. This does not mean that the risks can be ignored – the IPCC says “There is a high risk of substantial carbon and methane emissions as a result of permafrost thawing” [10]. Hence, the IPCC’s estimates of impacts are likely to be too low!


We have now seen what climate change is and how humans have contributed to it. In our next blog post, we will explore why we, as a species, continue to burn fossil fuels even though it is clearly so dangerous to our future survival.


[1] http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/grnhse.html

[2] http://www.giss.nasa.gov/research/briefs/ma_01/

[3] http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html – 100 year time horizon

[4] http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter02_FINAL.pdf

[5] http://ocean.nationalgeographic.com/ocean/critical-issues-sea-level-rise/

[6] http://ocean.si.edu/ocean-acidification

[7] http://www.metoffice.gov.uk/climate-change/guide/science/explained/feedbacks

[8] http://geology.gsapubs.org/content/32/1/53.short

[9] http://www.sciencemag.org/content/327/5970/1246.full

[10] http://www.ipcc.ch/pdf/assessment-report/ar5/syr/SYR_AR5_LONGERREPORT.pdf

[11] http://iopscience.iop.org/1748-9326/9/8/085003/pdf/1748-9326_9_8_085003.pdf

[12] http://www.climategeology.ethz.ch/publications/2013_Vaks_et_al.Scienceexpress.pdf

[13] http://www.nature.com/news/2005/050314/full/news050314-13.html

[14] http://earthobservatory.nasa.gov/Features/GlobalWarming/page3.php

[15] http://www.realclimate.org/index.php/archives/2013/09/what-ocean-heating-reveals-about-global-warming/