Climate Change: The Essentials

21 Dec 2005 - Special Reports, Atmosphere & Space, Water & Oceans, Land & Geology, Ice & Snow, Flora & Fauna, Arctic, Antarctic

Full Earth showing Africa and Antarctica - Copyright: NASA

Climate change linked to global warming is a critical issue for polar scientists and researchers. The climate of the poles is changing faster than the rest of the world and what once seemed immutable aspects of the polar climate, such as Arctic sea ice at the North Pole, are now under threat. It's also now well understood that the polar climates link strongly into the global climate system.

What happens at the poles affects us all. Understanding how the climate is changing there, from both natural and human-induced influences, and how it will affect other climate systems is a top priority for the polar scientific community.

The term "climate change" can be a little confusing as there are really two types of climate change:

there are the "natural" variations in the Earth's climate that have been going on throughout the planet's five billion year life and which have continued throughout the comparatively very short period of human existence; and
there is human-induced climate change.

Human-induced climate change largely results from greenhouse gases emitted through fossil fuel consumption since the industrial revolution of the nineteenth century. These emissions are considered by a vast majority of scientists to have driven an increase in the global average temperature - over and above changes which natural forces have induced.

Human-induced climate change is also referred to as "global warming", but the term climate change is used here in recognition of the fact that the impact of increased average temperature varies across the globe and affect a great range of climate phenomena.

The term "greenhouse effect" also has two senses:

the natural greenhouse effect is something extremely positive for the planet's biological organisms - the composition of the atmosphere and the presence of carbon dioxide keep the planet warm enough to sustain life; and
the "enhanced greenhouse effect" refers to the human-induced increase in the natural greenhouse effect as a result of increased greenhouse gas emissions.

The extent of the natural greenhouse effect has varied significantly over geological time as the composition of the atmosphere has also varied considerably. In the past there have been very warm periods where greenhouse gases, notably carbon dioxide, made up a much greater portion of the atmosphere. The potential problems we are now particularly concerned about arise from the human-induced enhancement of the greenhouse effect and from the pace at which they appear to be exacerbating climate change.

Humanity faces considerable challenges, first in slowing, then in stopping and reversing the accumulation of increasing quantities of greenhouse gases in the atmosphere. Economies have developed around the use of fossil fuels, whose large-scale consumption is primarily responsible for increasing greenhouse gas emissions. Finding less carbon-intensive paths for future global economic development will involve considerable investment of time, ingenuity and resources. It's a necessary investment, however, as the linked articles to this feature show.

Natural climate change over geological time

The Earth's climate has changed radically throughout its five billion year history as a result of changing natural forces operating on the planet's climate as a whole. In the past, temperatures have been considerably warmer than they are now and considerably cooler as well. Changes have generally taken place relatively slowly with a few notable exceptions.

Some may ask, on this basis, what the global warming fuss is about. It's important to recall that the planet's relatively cooler climate of the last few million years is what has proved conducive to the emergence of homo sapiens, and that modern, agriculture based civilisation has only flourished in the last ten thousand years or so. Were the more pessimistic forecasts of global warming of up to more than 5.8 degrees Celsius to materialise by the end of the century we'd be moving back to average temperatures not seen for over fifty million years. Much hotter temperatures might have suited the dinosaurs well enough but are unlikely to suit us! Moreover, shifting the planet's average temperature upwards by five degrees in a century is an extraordinarily fast change in climate. This would have enormous impacts on ecosystems around the world and very costly socio-economic impacts.

Climate forcings

There have been significant downswings (ice ages) and upswings (interglacials) in global average temperature in the past. These ice ages and interglacials can each last tens or hundreds of thousands of years.

Changing natural forces, often referred to as "climate forcings", drive natural climate change. Such change is due, in large part (but not exclusively), to:

the amount of heat generated by the sun (often following a regular pattern, including a short-term cyclical sunspot cycle);
variations in the earth's orbit around the sun and in the tilt and "wobble" of its axis (all of which follow reasonably predictable cycles measured in tens to hundreds of thousands of years);
the extent to which the Earth absorbs or reflects the sun's rays (known as albedo) - including through the size and positioning of ice caps and continents (they move around a lot over billions of years of geological time);
the extent to which the atmosphere has acted to transform solar radiation into heat as it reflects off the surface of the earth - otherwise known as "the greenhouse effect" - affected by both the atmosphere's gaseous composition (in particular, water vapour, carbon dioxide and methane) and by levels of particles in the air from volcanic activity.

The relative impact of a number of these forcings remains unclear and all of these factors can interact with each other in complex ways, but most scientists now consider levels of greenhouse gases in the atmosphere to be one of the dominant influences on global climate. In particular, the composition of the atmosphere is a critical determinant of how much solar radiation is converted into heat in the climate system - and links to a series of climate "feedback loops" relating, amongst other things, to the extent of vegetation on the continents and in the oceans. Such feedback loops are thought to have contributed to major, and sometimes relatively sudden, shifts in the planet's average temperature and climatic conditions.

A climate which balances itself?

The current climate (in geological terms, that of the last few million years) has been relatively predictable. Although there have been long glacial periods (ice ages) the warmer "interglacials" in between have come at reasonably measured intervals, approximately every 100,000 years. In large part, changes in the earth's orbit affecting the amount of heat the earth receives drive significant shifts to or from glacial periods. During the interglacial periods, including the one which we are now in (and which began around 12,000 years ago), the climate remains in a reasonably stable state. However, it does still vary enough to present considerable challenges for humanity and the biosphere (eg cooler temperatures during the "Little Ice Age" in 1400-1900, following the warmer "Medieval Optimum").

The stability of the climate system derives partly from a number of important auto-regulatory mechanisms that help maintain global average temperatures around the 15 degree Celsius mark. The following provides a brief account of the role of some of these key climate regulatory mechanisms.

Atmospheric circulation

Atmospheric circulation distributes heat from the equator to the poles. Each hemisphere has three linked systems of weather patterns which gradually move heat away from where the sun's rays have most impact (at the equator) towards where it has least impact (the Poles).

Thermohaline circulation

A vast system of ocean currents encircles the globe pushing heat outwards from the equator to the poles as well as pulling cold polar waters, at depth, to low latitudes. The Gulf Stream is part of this "thermohaline circulation", keeping temperatures in Western Europe far warmer on average than equivalent latitudes in North America. It is important to keep in mind that this is a long term process, distributing heat around the globe over a cycle which takes about 1000 years.

Albedo at the polar regions

The polar icecaps reflect considerable solar radiation: up to 90% depending on ice conditions, compared with much lower average albedos for other parts of the Earth's surface. This keeps the temperature at the poles lower than it would be without the ice cover.

Carbon and methane storage

The earth's vegetation cover is an important source of carbon storage: plants consume carbon dioxide, store carbon as they grow and also emit oxygen. Methane is stored when vegetable matter decays on the ocean floor (as submarine methane hydrates) and beneath permafrost (in tundra peat beds).These processes have helped keep overall greenhouse gas levels in the atmosphere in balance over time.

Greenhouse gases and global average temperatures

Scientific data indicates there is a close relationship between levels of greenhouse gases and global average temperatures. The higher the levels of greenhouse gases, the higher the global temperatures.

Over the last million years

Kilometres-long ice cores, recovered from Greenland and the Antarctic, have provided samples of actual atmosphere from over more than 900,000 years for scientists to analyse - thanks to bubbles of air trapped within the ice at the time of its formation.

What is immediately evident is that the levels of greenhouse gases have neither fallen below 180 parts per million nor exceeded 300 parts per million in the atmosphere until very recently.

It is also clear that the rises and falls in greenhouse gas levels have been closely correlated with the rises and falls in overall temperature levels (which were determined through examining the balance of two different oxygen isotopes within the atmospheric samples). While the cyclical changes in the planet's orbit have played the key role in inducing the cycle of rising and falling temperatures, there is no doubt that this range of 180 to 300 parts per million could be regarded as the planet's "comfort zone" with respect to its recent stable climatic state.

Samples of atmosphere from up to 900,000 years ago are currently being analysed. Early indications are that the same kind of relationship is evident in these even older ice archives.

Over the last thousand years

When we come to the time of human civilisation a different picture emerges. From 1860, as a result of the industrial revolution, there is a dramatic and continuously increasing rise in levels of CO2 as a result of the widespread uptake of fossil fuels as a source of heating, transportation and electricity generation.

Over the last two centuries we have moved outside the planet's comfort zone and are heading into uncharted territory. In 2005, levels reached around 380 parts per million, and certainly pose questions with regard to the planet's long term climate stability. It has been referred to as a "vast unsupervised experiment" on the planet (Elizabeth Kolbert, New Yorker, quoted in Bill Bryson, A Short History of Nearly Everything, p.520).

The Intergovernmental Panel on Climate Change (IPCC) was commissioned by the United Nations in the late 1980s to examine the question of how serious a problem climate change was. They have produced three reports since then. The vast majority of the over 2,000 respected scientists who took part in producing these reports have concluded that humanity is influencing the climate. One of the key factors behind their conclusion are the complex models which have been developed to simulate changes in the world's climate.

These models have been able to reproduce the historical global mean temperature record reasonably accurately for the first half of the 20th century, based on historical records of solar radiation, volcanic activity and other natural climate forcings. It is only possible to simulate global mean temperatures in the second half of this century if significant additional warming as a result of increasing levels of greenhouse gases, stemming from human activity, is factored in.

The Third Assessment Report released in 2001 looked at a number of scenarios of rising emissions levels over the coming century and concluded that greenhouse gas levels could rise to anywhere between 540 ppm and 970 ppm. Exactly where we end up on that scale will depend on the extent of economic growth and the extent to which it is fossil-fuel intensive (ie the extent to which we succeed in decarbonising economic growth).

The forecast levels of greenhouse gases are expected to correspond to increases in world mean temperature of between 1.4 and 5.8 degrees Celsius. Even if we stopped emitting greenhouse gases completely tomorrow, the temperature could be expected to continue to rise for several hundreds years, albeit more slowly.. Such increases will have significant impacts on the climate, but its extent will differ from region to region.

Global warming impacts and climate inertia

Overall impacts to date and forecast for the 21st century

The 2001 Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) concluded that the global average surface temperature increased by around 0.6 degrees over the last century and that the last decade was the warmest ever in the last 1000 years.

The report documented a rise in global average sea level and ocean heat content, and decreases in snow cover and ice extent both in mountain glaciers and Arctic sea ice. The IPCC report also pointed to increases in regional temperatures and observed changes in physical and biological systems in freshwater, terrestrial, and marine environments on most continents.

Using a range of emissions scenarios, the report's global average warming projections range from 1.4 to 5.8 ÂºC by 2100 relative to 1990. Global average sea level rise by 2100 could be from 9 to 88 cm, half of which comes from thermal expansion of sea water, about a quarter from melting glaciers and a small positive contribution from Greenland ice melt (offset in part by possible snow accumulation over Antarctica).

The report also stated there will likely be higher maximum temperatures for many land areas, and reduced frequency of low temperatures, including frosts. More intense precipitation events are likely over many mid to high-latitude land areas such as Europe, North America and Australia. Increased summer continental drying and associated risk of drought are likely in mid-latitudes. Tropical cyclones are projected to become more intense with higher peak winds and rainfall intensities.

Forecast Impacts at the Poles

The polar regions are particularly vulnerable to climate change impacts.

For example, a detailed study of the Arctic, released in 2004, confirmed that change was happening more rapidly in the Arctic than in other parts of the world. The Arctic Climate Impact Assessment Report (ACIA) indicated that the annual average Arctic temperature had increased at almost twice the rate as that of the rest of the world in recent decades.

The ACIA report also used IPCC projections of likely greenhouse gas emissions this century and concluded the Arctic would warm by 4-7 degrees (compared with the IPCC's lower global average range).

Such temperature increases would have very significant impacts on Arctic weather patterns and on ice, sea, soil, flora and fauna. A few of ACIA's more striking conclusions follow:

There will be substantial decreases in snow and ice cover (subsequent modelling by scientists has suggested that there will be little ice during summer in the Arctic Ocean by 2100). This will alter the Earth's albedo and will mean that fewer of the sun's rays will be reflected by the Earth's surface and more absorbed - creating a further warming effect.
The mix of species in the Arctic will change. Species from the south will move into previously uninhabitable terrain. Some existing Arctic species such as polar bears will be pushed toward extinction.
Thawing permafrost will disrupt transportation, buildings and other infrastructure.
Indigenous communities, already facing striking changes (for example the arrival of wasps for the first time!), will likely see serious challenges to health and food security with reduced numbers of local species such as polar bear, walrus, seals and caribou.

There are some changes which could be regarded as having some positive aspects. The treeline and even agriculture may move northwards where suitable soils exist and some arctic marine fisheries would likely become more productive. With the retreat of the ice, sea routes via the Arctic could be opened up. Such changes need to be kept in perspective, however. They would come at significant cost to other parts of local ecosystems and to the global climate as a whole.

Climate Inertia

Greenhouse gases infuse the atmosphere for a long time. Carbon dioxide persists for at least a century after it is emitted and some of it will last for several centuries. The greenhouse gas emissions of today will affect the climate experienced by many generations.

Climate systems take a long time to demonstrate the full extent of the impact of a warming planet. Thermohaline circulation turns over vast quantities of water and the water which "sinks" to the ocean floor as part of its conveyor-belt like overturning will take centuries to resurface elsewhere on the planet. This conveyor-belt, in part driven by the sinking of Arctic waters becoming colder, denser and saltier as it travels towards the Pole, could see disturbances which would only play out over many centuries.

So there is an urgent need to act now, but in the knowledge that efforts to change - both our emissions levels and the impacts that are already in the pipeline as a result of past fossil fuel consumption - will be like trying to slow, stop and then turn around a supertanker to avoid a collision. Foresight, energy and determination will be necessary to avoid a climate wreck.

By: Richard de Ferranti