Last edited 21 Jun 2021

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Climate change science


[edit] Introduction

Climate change is a particularly difficult problem for humankind to deal with. There are three aspects to the phenomenon that make it so.

First, to mitigate the effects of climate change means taking action now to avoid potentially catastrophic impacts that would take place far in the future – certainly far enough away to be beyond the political, economic and behavioural timescales our society is geared toward.

Second, should these impacts come to pass, it would mean that our mitigation efforts were lacking and at that point it would be too late to do anything about them. The inertia of the earth’s oceanic, atmospheric, cryospheric, biological and chemical systems mean that once ‘dangerous’ climate change is underway, there is not much we can do about it.

Third, these two truths are peppered with a degree of uncertainty around the timing and severity of the changes we may be exposed to. This is all to do with the earth’s ‘climate sensitivity’ – how it, as a system, will respond to the various forces it is being subjected to.

Despite some uncertainty around the severity of the temperature response to a given injection of carbon dioxide, and other greenhouse gases, into the atmosphere, the consensus is:

  • That the earth’s climate has always changed over timescales ranging from thousands of years to millennia; that alongside natural forcings.
  • Greenhouse gases from human activity are warming the world.
  • Effort is needed to reduce emissions and to adapt to the changes that are due to occur from the gases already injected into the atmosphere.

[edit] Climate science

There are a variety of forces, either internal or external to the earth system, that can have an impact on the climate – that is, the prevailing weather conditions in an area over a time period of around 30 years.

The climate clearly differs depending on the location on the earth’s surface and is in part a result of the redistribution of solar radiation from the equator towards the poles.

Climate is not a static entity. While weather might differ from day-to-day and year-to-year, the climate of a particular area will change over longer timescales. Evidence for this can be found in a variety of placesrecords of temperature, rain and wind data, for example.

When researchers are interested in finding out about the climate further back than is described in written records, proxy methods can be used to infer climatic conditions.

Tree rings (dendrochronology) can beused to provide a yearly record (one new ring per year) of what the climate was like in a particular area (wider rings indicate warmer years).

Pollen found in soil strata can indicate what sort of vegetation was around in a particular period, and by implication the conditions that allowed this vegetation to grow.

Gas levels in ice cores can give an indication of atmospheric composition, also referenced by year (new ice layers form year after year when snow and ice accumulates at the centre of ice sheets). There are many more methods.

Such proxies have allowed researchers to map the historic climate and how this has varied over different timescales.

A rhythm of climate change is set by the earth’s orbit around the sun. Milankovitch cycles, named after Milutin Milankovitch, a Serbian astronomer and mathematician who is credited with quantifying the phenomenon, relate to the earth’s movement around the sun in space. The tilt of the earth varies over time as well as the extent to which it wobbles on its axis. More fundamentally, the shape of the orbit (eccentricity) changes over time – from being more circular to more elliptical.

These orbital changes affect the amount of solar radiation reaching the earth and set the pace of glacial and interglacial periods in the earth’s climatic history to a 100,000-year timescale. The earth has gone from periods of being ice-free (interglacials) to being extensively covered by ice (glacials), and it is thought that these orbital changes act to kick off the changes between these two states.

These very long-term pulses have on top of them shorter-term fluctuations that affect the climate on shorter timescales and are related to other phenomenon. Currently the earth is in a relatively stable climatic period known as the Holocene which began 12,000 years ago – part of the current interglacial. But other natural factors have had an influence on the climate over this period.

The balance of radiation at and around the earth’s surface can be affected by forces either outside or inside the earth system.

[edit] Sunspots

Externally, the level of solar output varies due to sunspots, for example, and this will affect the amount of outgoing shortwave radiation that makes its way from the sun to the earth.

[edit] Volcanoes

Volcanic eruptions are one classic example, this time the result of the geological system, of an internal force affecting the amount of radiation reaching the earth’s surface. The earth’s radiation balance is affected by the composition of chemicals and particles volcanoes spew into the atmosphere. Global temperatures will drop by up to half a degree for a few years after a big eruption due to sulphate aerosols reflecting a greater amount of the earth’s radiation back to space. Volcanoes also emit greenhouse gases, but at a much lower level than the contribution made by human activity.

[edit] Clouds

Clouds also have an impact on the earth’s surface radiation balance. As well as reflecting shortwave radiation from the sun, they have the effect of absorbing and re-emitting long wave radiation from the earth’s surface. It is thought that clouds have an overall net cooling effect, but how this will change in the future with changes in atmospheric water vapour content, and changes in cloud amount and distribution, is a complex issue.

[edit] Greenhouse gases

Despite orbiting at twice the distance from the sun than its neighbour Mercury, Venus has a much higher average surface temperature – because it has an atmosphere; one that is largely made up of carbon dioxide.

Venus is an example of a planet that has undergone runaway global warming. While the temperature on Mercury reaches 426°C on its sunward side but falls to -173°C in the shadows, Venus stays at 462°C no matter the location. Venus’ atmospheric carbon dioxide absorbs and re-emits the long-wave radiation from the planet’s surface, preventing it from escaping into space and so warming the atmosphere.

Theory around the atmospheric greenhouse effect has been around since the 1800s. The work of Joseph Fourier in 1824 and then John Tyndall in 1864 helped to first show that the earth’s atmosphere acts to trap long-wave radiation and thus warm the planet, and that there are some gases that are more effective at doing this than others.

The Swedish physicist Svante Arrhenius later used observations of infrared absorption to estimate the effects of lowering the level of carbon dioxide in the earth’s atmosphere. He estimated that halving the atmospheric composition of carbon dioxide would not prompt the onset of an ice age, but, importantly, that doubling atmospheric carbon dioxide levels would warm the planet by a total of 5-6°C. This estimate is within the range of warming predicted by modern day climate models.

Humankind is currently putting these theories to the test, by conducting a planet-wide experiment in which greenhouse gas is injected into the atmosphere in ever increasing quantities.

Today, 110m tonnes of carbon dioxide enter the atmosphere every 24 hours as a result of human activities. The concentration of carbon dioxide in the atmosphere hit 400 parts per million (ppm) earlier this year, and continues on its upward trend. This concentration has increased from around 280 ppm before the industrial revolution. Some have argued that we actually needed to reduce this level to below 350 ppm to avoid irreversible and catastrophic climate change.

Though it is the most ubiquitous greenhouse gas related to human activities, carbon dioxide is not the only one. Methane is far more potent, alongside nitrous oxide, CFCs and even water vapour.

These gases are compared to carbon dioxide by their global warming potential (GWP), with carbon dioxide having a GWP of 1. Over 20 years in the atmosphere methane has a GWP of 86, and over 100 years 34 (the time horizon has an effect on the potency of the gas relative to carbon dioxide). CFCs are also very potent greenhouse gases, but are released into the atmosphere in relatively small amounts compared to carbon dioxide and methane.

Focusing on carbon, the natural carbon cycle sees the element move around the earth in various states as a result of natural processes. Levels of carbon dioxide in the atmosphere vary over the year as the earth’s flora ‘breathes in and out’ over growth and decay cycles. However, humans are taking carbon that has long been locked away, out of the system, and are converting it into carbon dioxide at a rate that the earth has never seen before – to levels not seen in the last million years.


Observed temperature change across the earth’s surface from 1901 to 2012. (Source: IPCC)

We have a climate system that responds to a host of natural, and now also anthropogenic forces. Both sides will have an influence on future climate.

[edit] Anthropogenic effects

Early indications of anthropogenic effects have already been seen. Fourteen of the last fifteen years broke world temperature records. After what some described as a ‘pause’ in the rate of surface temperature warming (studies have shown the world’s oceans absorbed more heat over the period), 2014 was crowned the warmest year on record by the US National Oceanic and Atmospheric Administration. The Intergovernmental Panel on Climate Change (IPCC) estimates a warming of 0.85°C from 1880 to 2012.

Some also argue that current extreme weather events are a result of climate change. In August 2015, for the first time ever, three ‘category four’ hurricanes were recorded over the Pacific Ocean simultaneously. Large heatwaves have occurred this year in India and Pakistan. California is experiencing long-lasting drought. Desertification is a growing issue. Speaking in London in September 2015, former US Vice President Al Gore said: ‘Every day the news is like a nature hike through the Book of Revelation.’

Computer climate models are used to predict future climate change (though some criticise them, perhaps overly scrupulously). Meta analyses average out a series of model predictions for researchers to get an average picture of how the climate may change in the future.

Climate models can also be used to hindcast past climate changes. As the IPCC has noted in the past, and in its most recent assessment report (AR5), models can’t replicate the observed temperature changes seen since 1951 when the models are spun up with natural forces and internal variability alone. Only when models take account of anthropogenic forcing do they match the observed record (though there has been a divergence over the last few years due to the models having trouble with predicting chaotic events).

The consensus is that the climate is changing, that humans are now the dominant factor in this change, and that we are locked into a certain amount of warming due to the greenhouse gases we have already emitted. But the complexity of the earth’s chaotic systems means that how it will respond to this forcing is uncertain.

[edit] Climate sensitivity

A number of comprehensive ocean-atmospheric global circulation climate models are used by various organisations to model future global climate trajectories. These can look at outputs such as temperature, precipitation or sea ice extent in certain areas as well as producing global average temperature paths.

Different models have different parameters and so a range of outcomes can occur depending on the underlying scenario on emissions and the timing of when emissions cuts take place, for example.

Key to the impacts of climate change is how the earth responds to greenhouse gases – or the level of the earth’s‘climate sensitivity’. This can be expressed as the level of eventual global average temperature increase experienced by a given increase in carbon dioxide into the atmosphere. A doubling of carbon dioxide from pre-industrial levels (280 ppm) is used to compare model responses.

The climate sensitivity is uncertain. It is almost certainly not below 1°C for a doubling of carbon dioxide, and it is unlikely to be above 6°C. The IPCC has slightly changed its wording on climate sensitivity in its recent reports, and in AR5 it says with medium confidence that it is likely to be between 1.5°C and 4.5°C. Quite a difference in temperature, and thus the associated impacts.


Change in global mean surface temperature relative to 1986–2100 for two IPCC scenarios – red largely represents business as usual while the blue scenario represents action on emissions. The shaded areas represent the uncertainty around the projections from the host of climate models that generated the curves. (Source: IPCC)

[edit] Feedbacks and the 2°C target

The phenomenon of feedbacks in the climate system is hugely important when it comes to climate and environmental change.

In general terms, negative feedbacks act to dampen the effect of an initial stimulus and can reverse a certain trend. The problem with many climate-related feedbacks is that they are largely positive in nature – they will act to enhance and amplify the effect that was already underway, thus feeding back and intensifying the initial direction of change. Uncertainty around how much of a role feedbacks will play in climate change is part of the reason for the range of climate sensitivity estimates.

A few good examples of climate feedbacks can be found in the upper northern hemisphere, in the Arctic.

[edit] Melting ice

The albedo effect is one. It relates to the amount of shortwave radiation that is reflected by a given surface on the earth. Lighter surfaces such as snow and ice reflect radiation back to the atmosphere and act to perpetuate a cooler local environment. Darker surfaces absorb heat and warm the surrounding area.

As Arctic sea ice and ice on land, for example, on the fringes of Greenland, melts due to increasing temperatures in the Arctic, this exposes the darker underlying sea or land. This results in further local warming, reinforcing the original stimulus.

The rate of rapid decline of Arctic sea ice extent over the last 30 years has taken many by surprise, and is partly a result of this feedback. Estimates of when the Arctic will be free of sea ice have been brought forward significantly (which in itself will then feed back into the climate system).

[edit] Freshwater

Further feedbacks abound. The injection of more freshwater into the Arctic Sea as a result of this melting could disrupt the ocean’s thermohaline circulation, which distributes heat around the globe, thus having climatic effects further afield.

Melting permafrost could release vast stores of trapped methane into the atmosphere – clearly not a good thing. The Greenland ice sheet could reach a melting point where internal cryospheric mechanisms are initiated to result in its total, unstoppable collapse, resulting in a 7 m increase in global sea level.

These are the sorts of things touched on when people talk about irreversible, dangerous and runaway climate change. And as discussed earlier, theory says that once certain ‘tipping points’ are passed, the earth’s climate system could flip into an alternate permanent state that could be detrimental to current life on earth.

So, the climate sensitivity is fundamental to whether we may be able to adapt to the consequences of the greenhouse gases we have emitted and are locked into emitting, of whether we may be exposed to environmental turmoil.

This has led to the well-known goal to limit warming to a 2°C rise over pre-industrial temperatures to avoid ‘dangerous’ climate change. This was decided in the 1990s to be an adequate target, but many have argued subsequently that it will still come with significant impacts.

Many in the climate community now regard keeping to below 2°C as unrealistic due to the carbon emissions we are ‘locked into’ under business as usual. The IPCC says it is likely that 450 ppm of carbon dioxide will equate to 2°C, but others argue that we shouldn’t go past 400 ppm (which was reached earlier this year), or even 350 ppm.

[edit] How much more carbon can we emit?

In AR5, the IPCC for the first time outlined a ‘carbon budget’ – the amount of carbon we can still emit and keep a reasonable chance (66%) of staying below 2°C. Accounting for other warming factors, it set this as a total of 800bn tonnes of carbon on top of pre-industrial levels. We have already emitted around 530bn tonnes, meaning about two thirds of the budget has gone. At current emission rates we will blow the budget in 25 years.

AR5 also stresses the importance of cumulative emissions – i.e. it is better from a total emissions point of view to follow a trajectory of emissions reductions where steeper cuts are made in earlier years than left till the last minute, as the net reduction would be much greater.

Overshooting carbon dioxide targets and then having to take carbon back out of the atmosphere is an option, but geoengineering techniques are viewed with suspicion and many deride them as a bad idea. The IPCC does however explore the ‘Plan B’ option of burning biomass in power stations and then capturing and storing the carbon to produce negative emissions.

Staying below 2°C is seen as a significant challenge. The IPCC says that to do so we need to rapidly deploy a significant additional amount of low carbon power generation and prove carbon capture and storage (CCS) quickly. Bioenergy and CCS (BECCS) may then have a part to play. Most IPCC 2°C scenarios feature some form of BECCS element.

Anthropogenic climate change is indeed a detailed and difficult problem, but what underlies it all? Is it an expression of a deep part of human behaviour in which individual short-termism will always triumph over collective action? That certainly is part of the reason we are in this situation.

But collectively we are taking action by pushing forward low carbon energy solutions and measures on deforestation. We have an innate desire to protect fragile ecosystems and endangered species.

The case for action is clear. The challenge is significant, but a positive outcome is possible. Our way of life depends on it.

This article was written by--Marc Height 11:12, 04 Dec 2015 (BST)

First published in the November 2015 issue of Energy World.

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