Frequently asked questions on climate change

Climate change research runs across many different disciplines from natural sciences, engineering and technologies to the social and economic fields. As such, it generates many questions from peope trying to gain a better understanding of the area.   Below are a list of frequently asked questions (FAQs) on climate change to help you understand the topic in more detail. Sources for the answers include the Intergovernmental Panel on Climate Change, the European Environment Agency and www.change.ie.

How will climate change affect Ireland?

Analysis of meteorological data for Ireland shows that the climate has changed over the past 100 years. This change is similar to regional and global patterns as reported in the Integrated Panel on Climate Change Assessment Report 4  (IPCC AR4). The clearest trend is evident in the temperature records but there is also a trend towards more intense and frequent rainfall. Some of the indicators of climate change in Ireland include:

  • Ireland’s mean annual temperature increased by 0.7 degrees celcius (°C) between 1890 and 2004.
  • The average rate of increase is 0.06°C per decade. However, as Ireland experiences considerable climate variability, the trend is not linear.
  • The highest ten-yearly rates of increase have occurred since 1980, with a warming rate of 0.42°C per decade.
  • Six of the 10 warmest years have occurred since 1990. Our climate will continue to warm with possible increases of 3º to 4ºC towards the end of the century.
  • There has been a reduction in the number of frost days and a shortening of the frost season length.
  • The annual precipitation has increased on the north and west coasts, with decreases or small increases in the south and east.


These changes are reflected in ecosystem changes, with increase in the growing season and greater numbers of warmer latitude fauna being evident in Ireland and its surrounding waters.

Climate change impacts are projected to increase in the coming decades and during the rest of this century. Uncertainties remain in relation to the magnitude and extent of these impacts, particularly during the second half of the century. The greatest uncertainly lies in how effective global actions will be in reducing greenhouse gas emissions. Predicted negative changes include:

  • the pace of sea-level rise, which has increased from 1.8 millimetres per annum (mm/a) in 1961 to 3 mm/a in 2005, resulting in a projected sea level rise of between 18 cm and 59 cm this century
  • more intense storms and rainfall events
  • increased likelihood and magnitude of river and coastal flooding; increased storm surges
  • water shortages in summer in the east, need for irrigation of crops
  • negative impacts on water quality
  • changes in the distribution of species, and possible extinction of vulnerable species requiring cooler conditions, e.g. the Arctic char
  • effects on fisheries that are sensitive to small changes in temperature, e.g. cod
  • increased frequency of wild fires and pest infestation.

(Source: EPA Climate Change Research Programme)

For more information on this topic please see the EPA climate change research programme

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What factors determine Earth's climate?

The climate system is a complex, interactive system consisting of the atmosphere, land surface, snow and ice, oceans and other bodies of water, and living things. The atmospheric component of the climate system most obviously characterises climate.

Climate is often defined as ‘average weather’.

Climate is usually described in terms of the mean and variability of temperature, precipitation and wind over a period of time, ranging from months to millions of years (the classic period is 30 years).

The climate system evolves in time under the influence of its own internal dynamics and due to changes in external factors that affect climate (called ‘forcings’). External forcings include natural phenomena such as volcanic eruptions and solar variations, as well as human-induced changes in atmospheric composition. Solar radiation powers the climate system. There are three fundamental ways to change the radiation balance of the Earth:

1. by changing the incoming solar radiation (e.g., by changes in Earth’s orbit or in the Sun itself);

2. by changing the fraction of solar radiation that is reflected (called ‘albedo’; e.g., by changes in cloud cover, atmospheric particles or vegetation);

3. by altering the longwave radiation from Earth back towards space (e.g., by changing greenhouse gas concentrations)

Climate, in turn, responds directly to such changes, as well as indirectly, through a variety of feedback mechanisms. The figure below displays the various sources of incoming and outgoing radiation to and from the earth.

 This image shows various sources of incoming and outgoing radiation to / from the earths surface.
Source: Figure 1 FAQ 1.1 Intergovernmental Panel  FAQ 1.1- Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007)

For more information on this topic, see FAQ 1.1, extracted from Chapter 1 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007)

What is the relationship between climate change and weather?

Climate is generally defined as average weather, and as such, climate change and weather are intertwined. Observations can show that there have been changes in weather, and it is the statistics of changes in weather over time that identify climate change. While weather and climate are closely related, there are important differences.

A common confusion between weather and climate arises when scientists are asked how they can predict climate 50 years from now when they cannot predict the weather a few weeks from now. The chaotic nature of weather makes it unpredictable beyond a few days. Projecting changes in climate (i.e., long-term average weather) due to changes in atmospheric composition or other factors is a very different and much more manageable issue. As an analogy, while it is impossible to predict the age at which any particular man will die, we can say with high confidence that the average age of death for men in industrialised countries is about 75.

Another common confusion of these issues is thinking that a cold winter or a cooling spot on the globe is evidence against global warming. There are always extremes of hot and cold, although their frequency and intensity change as climate changes. But when weather is averaged over space and time, the fact that the globe is warming emerges clearly from the data.

This figure shows a picture of the air the sea and land and the climate interactions between them

For more information on this topic, see FAQ 1.2, extracted from Chapter 1 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

What is the greenhouse effect?

The Sun powers Earth’s climate, radiating energy at very short wavelengths, predominately in the visible or near-visible (e.g., ultraviolet) part of the spectrum. Roughly one-third of the solar energy that reaches the top of Earth’s atmosphere is reflected directly back to space. The remaining two-thirds is absorbed by the surface and, to a lesser extent, by the atmosphere. To balance the absorbed incoming energy, the Earth must, on average, radiate the same amount of energy back to space. Because the Earth is much colder than the Sun, it radiates at much longer wavelengths, primarily in the infrared part of the spectrum (see Figure 1).

 

This figure shows an idealised model for the greenhouse effect. The earths atmosphere and the earths surface are shown with solar radiation being absorbed an reflected by same.

Figure 1: An idealised model for the Greenhouse gas effect (Source IPCC 2007)

Much of this thermal radiation emitted by the land and ocean is absorbed by the atmosphere, including clouds, and reradiated back to Earth. This is called the greenhouse effect. The glass walls in a greenhouse reduce airflow and increase the temperature of the air inside. Analogously, but through a different physical process, the Earth’s greenhouse effect warms the surface of the planet. Without the natural greenhouse effect, the average temperature at Earth’s surface would be below the freezing point of water. Thus, Earth’s natural greenhouse effect makes life as we know it possible. However, human activities, primarily the burning of fossil fuels and clearing of forests, have greatly intensified the natural greenhouse effect, causing global warming.

For more information on this topic, see FAQ 1.3, extracted from Chapter 1 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007)

 

 

 

 

 

 

What are the main Greenhouse Gases?

A wide range of gases known as greenhouse gases contribute to global warming/climate change. The most important ‘greenhouse gases’ are:

  • carbon dioxide  (CO2
  • methane (CH4
  • nitrous oxide (N2O)


Other greenhouse gases include the F- Gases:  hydrofluorocarbons (HFC), perfluorocarbons (PFC) and sulphur hexafluoride (SF6).

(Source: www.change.ie)

 

 

 

What are the Global Warming Potential (GWP) of the different gases?

The different gases have different atmospheric characteristics, including global warming potentials (GWP). The GWP of a gas is a measure of the cumulative warming over a specified time period usually 100 years, by a unit mass of this gas. This is expressed relative to carbon dioxide (CO2) which has a GWP of 1. The mass emission of any gas multiplied by its GWP gives the equivalent emission of the gas as carbon dioxide. This is known as CO2 equivalent. This makes it easier to sum up the emissions and contribution of Greenhouse Gas (GHG) to climate change and determine options to address climate change.

Therefore, while carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are important because they are normally emitted in large amounts, the fluorinated gases, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6) are also important mainly because of their comparatively much larger GWP values.
 
The atmospheric lifetime of greenhouse gases is another key factor in determining what actions are required to combat climate change. Carbon dioxide is considered to stay in the atmosphere for 100 years. Other greenhouse gases have much longer lifetimes, for example sulphur hexafluoride has a lifetime of 3200 years. Methane has a relatively short lifetime of 12 years. These long lifetimes in the atmosphere mean that emissions today will have an impact on the climate for the rest of this century and, in the case of some gases, beyond. The long life of these gases is one of the reasons that action on reducing emissions is so vital to prevent long term climate change.

(Source www.Change.ie)

How does human activity lead to additional greenhouse gases?

Greenhouse gases (GHGs) are emitted by numerous activities – the main greenhouse gas is carbon dioxide (CO2), which arises from the burning of fossil fuels and land use changes. Other GHGs include methane (CH4), from agriculture and waste food and nitrous oxide (N2O), mainly arising from agriculture. Industrial gases such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6) also act as very powerful greenhouse gases but are emitted in much smaller quantities.

Generally the sources of emissions can be broadly divided into two categories:

(1) energy related

(2) non-energy related.

Energy related emissions arise through power generation, transport, industry, and buildings (heating and other fuel use).

Non-energy related emissions arise from agriculture, forestry, land use change and waste disposal activities.

The cycle of greenhouse gases are part of life on earth. However, it is the enhanced levels of emissions of these gases that is currently part of modern life which we need to address. If we are to effectively reduce current excessive emissions of greenhouse gases, fundamental changes are required in the way energy is produced and consumed, work is organised, leisure and travel and management of land and forests.

(Source: www.change.ie)

Is sea level rising?

Yes, there is strong evidence that global sea level gradually rose in the 20th century and is currently rising at an increased rate, after a period of little change between AD 0 and AD 1900. Sea level is projected to rise at an even greater rate in this century. The two major causes of global sea level rise are thermal expansion of the oceans (water expands as it warms) and the loss of land-based ice due to increased melting. 
 This graph shows the sea level change in millimetres from the year 1800 through to predictions to 2100
Figure 1. Time series of global mean sea level in the past and as projected for the future (Source: IPCC, 2007)

For more information on this topic, see FAQ 5.1, extracted from Chapter 4 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007)

Is the amount of snow and ice on the Earth decreasing?

Yes. Observations show a global-scale decline of snow and ice over many years, especially since 1980 and increasing during the past decade, despite growth in some places and little change in others. Most mountain glaciers are getting smaller. Snow cover is retreating earlier in the spring. Sea ice in the Arctic is shrinking in all seasons, most dramatically in summer. Reductions are reported in permafrost, seasonally frozen ground and river and lake ice. Important coastal regions of the ice sheets on Greenland and West Antarctica, and the glaciers of the Antarctic Peninsula, are thinning and contributing to sea level rise. The total contribution of glacier, ice cap and ice sheet melt to sea level rise is estimated as 1.2 ± 0.4 mm/yr for the period 1993 to 2003.

For more information on this topic, see FAQ 4.1 , extracted from Chapter 4 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007)

Has there been a change in extreme events like heat waves, droughts, floods and hurricanes?

Since 1950, the number of heat waves has increased and widespread increases have occurred in the numbers of warm nights. The extent of regions affected by droughts has also increased as precipitation over land has marginally decreased while evaporation has increased due to warmer conditions. Generally, numbers of heavy daily precipitation events that lead to flooding have increased, but not everywhere.

Tropical storm and hurricane frequencies vary considerably from year to year, but evidence suggests substantial increases in intensity and duration since the 1970s. In the extratropics, variations in tracks and intensity of storms reflect variations in major features of the atmospheric circulation, such as the North Atlantic Oscillation.

How is precipitation changing?

Observations show that changes are occurring in the amount, intensity, frequency and type of precipitation. These aspects of precipitation generally exhibit large natural variability, and El Niño and changes in atmospheric circulation patterns such as the North Atlantic Oscillation have a substantial influence.

Pronounced long-term trends from 1900 to 2005 have been observed in precipitation amount in some places: significantly wetter in eastern North and South America, northern Europe and northern and central Asia, but drier in the Sahel, southern Africa, the Mediterranean and southern Asia. More precipitation now falls as rain rather than snow in northern regions. Widespread increases in heavy precipitation events have been observed, even in places where total amounts have decreased.

These changes are associated with increased water vapour in the atmosphere arising from the warming of the world’s oceans, especially at lower latitudes. There are also increases in some regions in the occurrences of both droughts and floods.

For more information on this topic, see FAQ 3.2 , extracted from Chapter 3 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

What caused the Ice Ages and other important climate changes before the industrial era?

Climate on Earth has changed on all time scales, including long before human activity could have played a role. Great progress has been made in understanding the causes and mechanisms of these climate changes. Changes in Earth’s radiation balance were the principal driver of past climate changes, but the causes of such changes are varied. For each case – be it the Ice Ages, the warmth at the time of the dinosaurs or the fluctuations of the past millennium – the specific causes must be established individually. In many cases, this can now be done with good confidence, and many past climate changes can be reproduced with quantitative models.

For more information on this topic, see  FAQ 6.1 , extracted from Chapter 6 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007)


Do projected changes in climate vary from region to region?

Climate varies from region to region. This variation is driven by the uneven distribution of solar heating, the individual responses of the atmosphere, oceans and land surface, the interactions between these, and the physical characteristics of the regions. The perturbations of the atmospheric constituents that lead to global changes affect certain aspects of these complex interactions.

Some human-induced factors that affect climate (‘forcings’) are global in nature, while others differ from one region to another. For example, carbon dioxide, which causes warming, is distributed evenly around the globe, regardless of where the emissions originate, whereas sulphate aerosols (small particles) that offset some of the warming tend to be regional in their distribution.

Furthermore, the response to forcings is partly governed by feedback processes that may operate in different regions from those in which the forcing is greatest. Thus, the projected changes in climate will also vary from region to region.

This figure shows projected changes in global rainfall to the en dof the century in different months of the year
(Source: Figure 1 FAQ 11.1 Intergovernmental Panel  FAQ 11.1- Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007)

Figure 1. Blue and green areas on the map are by the end of the century projected to experience increases in precipitation, while areas in yellow and pink are projected to have decreases. The top panel shows projections for the period covering December, January and February, while the bottom panel shows projections for the period covering June, July and August

For more information on this topic, see FAQ 11.1, extracted from Chapter 11 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physical Science Basis (2007).

 

If emissions of greenhouse gases are reduced, how quickly do their concentrations in the atmosphere decrease?

The adjustment of greenhouse gas concentrations in the atmosphere to reductions in emissions depends on the chemical and physical processes that remove each gas from the atmosphere. Concentrations of some greenhouse gases decrease almost immediately in response to emission reduction, while others can actually continue to increase for centuries even with reduced emissions. See Figure 1 below.


Figure 1.
(a) Simulated changes in atmospheric CO2 concentration relative to the present-day for emissions stabilised at the current level (black), or at 10% (red), 30% (green), 50% (dark blue) and 100% (light blue) lower than the current level; (b) as in (a) for a trace gas with a lifetime of 120 years, driven by natural and anthropogenic fluxes; and (c) as in (a) for a trace gas with a lifetime of 12 years, driven by only anthropogenic fluxes. (Source IPCC, 2007)

For more information on this topic, see FAQ 10.3, extracted from Chapter 3 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

Can individual extreme events be explained by greenhouse warming?

Changes in climate extremes are expected as the climate warms in response to increasing atmospheric greenhouse gases resulting from human activities, such as the use of fossil fuels. However, determining whether a specific, single extreme event is due to a specific cause, such as increasing greenhouse gases, is difficult, if not impossible, for two reasons:

1. extreme events are usually caused by a combination of factors;

2. a wide range of extreme events is a normal occurrence even in an unchanging climate.

Nevertheless, analysis of the warming observed over the past century suggests that the likelihood of some extreme events, such as heat waves, has increased due to greenhouse warming, and that the likelihood of others, such as frost or extremely cold nights, has decreased. For example, a recent study estimates that human influences have more than doubled the risk of a very hot European summer like that of 2003.

For more information on this topic, see  FAQ 9.1 , extracted from Chapter 9 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physical Science Basis (2007).

How do people adapt to climate change?

Current adaptation strategies
Humans need to adapt to the impacts of climate change, for instance through technological solutions such as coastal defences and changes in consumption habits. Humans are already adapting to climate change, and further adaptation efforts will be necessary during coming decades. However, adaptation alone is not expected to be able to cope will all projected effects since the options diminish and the costs increase with rising temperatures.

Vulnerability of populations to climate change
Vulnerability of human populations to climate change and its consequences can be affected by other factors, such as pollution, conflicts, or epidemics such as AIDS. An emphasis on sustainable development can help human societies reduce their vulnerability to climate change. However, climate change itself can become an impediment to their development.

Mitigation and adaptation

Mitigation measures that aim to reduce greenhouse gases emissions can help avoid, reduce or delay impacts, and should be implemented in order to ensure that adaptation capacity is not exceeded.

(Source: European Environment Agency)

What impacts of climate change have already been observed?

Regional climate change is already affecting many natural systems. For instance, it is increasingly being observed that snow and ice are melting and frozen ground is thawing, hydrological and biological systems are changing and in some cases being disrupted, migrations are starting earlier, and species' geographic ranges are shifting towards the poles.

Despite remaining gaps in knowledge, it is likely that these effects are linked to human influence on climate. At the regional level, however, responses to natural variability are difficult to separate from the effects of climate change.

Some previously unanticipated impacts of regional climate change are just starting to become apparent. For instance, melting glaciers can threaten mountain settlements and water resources, and damage associated with coastal flooding are increasing.

To find out more about climate change in Ireland, download the the EPA report Summary of the state of knowledge on climate change impacts for Ireland.

What actions can be taken to reduce greenhouse gas emissions?

Cost of Mitigation

Mitigation measures to reduce greenhouse gas emissions have a certain cost. However, they also constitute an economic benefit by reducing the impacts of climate change, and the costs associated with them. In addition, they can bring economic benefits by reducing local air pollution and energy resource depletion.

If the benefits of avoided climate change are taken into account and a "carbon price" is established for each unit of greenhouse gas emissions, this could create incentives for producers and consumers to significantly invest in products, technologies and processes which emit less greenhouse gases. The resulting mitigation potential is substantial and could offset the projected growth of global emissions over the coming decades or reduce emissions below current levels.

Mitigation measures could contribute to stabilizing the concentration of greenhouse gases in the atmosphere by 2100 or later. To achieve low stabilization levels, stringent mitigation efforts are needed in the coming decades. This could reduce global GDP by up to a few percent.

Changes in lifestyle and behaviours

Changes in lifestyle and behaviours that favor resource conservation can contribute to climate change mitigation.

Co-benefits of mitigation

Mitigation measures can also have other benefits for society, such as health cost savings resulting from reduced air pollution. However, mitigation in one country or group of countries could lead to higher emissions elsewhere or effects on the global economy.

Reduction potential of different sectors

No one sector or technology can address the entire mitigation challenge. All sectors including buildings, industry, energy production, agriculture, transport, forestry, and waste management could contribute to the overall mitigation efforts, for instance through greater energy efficiency. Many technologies and processes which emit less greenhouse gases are already commercially available or will be in the coming decades.

Longer term implications of mitigation actions

In order to stabilize the concentration of greenhouse gases in the atmosphere, emissions would have to stop increasing and then decline. The lower the stabilization level aimed for, the more quickly this decline would need to occur. World-wide investments in mitigation technologies, as well as research into new energy sources, will be necessary to achieve stabilization. Delaying emission reduction measures limits the opportunities to achieve low stabilization levels and increases the risk of severe climate change impacts.

How likely are major or abrupt climate changes, such as loss of ice sheets or changes in global ocean circulation?

Abrupt climate changes, such as the collapse of the West Antartic Ice Sheet, the rapid loss of the Greenland ice sheet or large-scale changes of ocean circulation systems, are not considered likley to occur in the 21st  century, based on currently available model results. However, the occurrence of such changes becomes increasingly more likely as the perturbation of the climate system progresses.

For more information on this topic, see FAQ 10.2, extracted from Chapter 10 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

Are extreme events, like heat waves, droughts or floods, expected to change as the Earth's climate changes?

Yes; the type, frequency and intensity of extreme events are expected to change as Earth’s climate changes, and these changes could occur even with relatively small mean climate changes. Changes in some types of extreme events have already been observed, for example, increases in the frequency and intensity of heat waves and heavy precipitation events.

For more information on this topic, see FAQ 10.1 extracted from Chapter 10 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

Can the warming of the 20th century be explained by natural variability?

It is very unlikely that the 20th century warming can be explained by natural causes. The late 20th century has been unusually warm. Palaeoclimatic reconstructions show that the second half of the 20th century was likely the warmest 50-year period in the Norther Hemisphere in the last 1300 years. This rapid warming is consistent with the scientific understanding of how the climate should respond to a rapid increase in greenhouse gases like that which has occured over the past century, and the warming is inconsistent with the scientific understanding of how the climate should respond to natural external factors such as the variability in solar output and volcanic activity.

Climate models provide a suitable tool to study the various influences on the Earth's climate. When the effects of increasing levels of greenhouse gases are included in the models, as well as natural external factors, the models produce good simulations of warming that has occured over the past century. The models fail to produce the observed warming when run using only natural factors. When human factors are included, the models also simulate a geographic pattern of temperature change around the globe similar to that which has occured in recent decades. This spatial pattern, which has features such as greater warming at higer northern latitudes, differs from the most important patetrns of natural climate variability that are associated with internal climate processes, such as El Nino.

 Source: Figure 1 FAQ 9.2 Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007)

(Source: Figure 1 FAQ 9.2 Intergovernmental Panel  FAQ 9.2- Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007) 

For more information on this topic, see FAQ 9.2, extracted from Chapter 9 of the Intergovernmental Panel on Climate Change 4th Assesment Report, Working Group 1, The Physicals Science Basis (2007).

 

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How reliable are the models used to make projections of future climate change?

There is considerable confidence that climate models provide credible quantitative estimates of future climate change, particularly at continental scales and above. This confidence comes from the foundation of the models in accepted physical principles and from their ability to reproduce observed features of current climate and past climate changes. Confidence in model estimates is higher for some climate variables (e.g., temperature) than for others (e.g., precipitation). Over several decades of development, models have consistently provided a robust and unambiguous picture of significant climate warming in response to increasing greenhouse gases.

For more information on this topic, see FAQ 8.1, extracted from Chapter 8 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

Are the increases in atmospheric carbon dioxide and other greenhouse gases during the industrial era caused by human activities?

Yes, the increases in atmospheric carbon dioxide (CO2) and other greenhouse gases during the industrial era are caused by human activities. In fact, the observed increase in atmospheric CO2 concentrations does not reveal the full extent of human emissions in that it accounts for only 55% of the CO2 released by human activity since 1959. The rest has been taken up by plants on land and by the oceans. In all cases, atmospheric concentrations of greenhouse gases, and their increases, are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound).

Fossil fuel combustion (plus a smaller contribution from cement manufacture) is responsible for more than 75% of human-caused CO2 emissions. Land use change (primarily deforestation) is responsible for the remainder. For methane, another important greenhouse gas, emissions generated by human activities exceeded natural emissions over the last 25 years.

For nitrous oxide, emissions generated by human activities are equal to natural emissions to the atmosphere. Most of the long-lived halogen-containing gases (such as chloro-fluorcarbons) are manufactured by humans, and were not present in the atmosphere before the industrial era.

On average, present-day tropospheric ozone has increased 38% since pre-industrial times, and the increase results from atmospheric reactions of short-lived pollutants emitted by human activity.

The concentration of CO2 is now 379 parts per million (ppm) and methane is greater than 1,774 parts per billion (ppb), both very likely much higher than any time in at least 650 kyr (during which CO2 remained between 180 and 300 ppm and methane between 320 and 790 ppb). The recent rate of change is dramatic and unprecedented; increases in CO2 never exceeded 30 ppm in 1 kyr – yet now CO2 has risen by 30 ppm in just the last 17 years.


For more information on this topic, see FAQ 7.1 , extracted from Chapter 7 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

Is the current climate change unusual compared to earlier changes in Earth's history?

Climate has changed on all time scales throughout Earth’s history. Some aspects of the current climate change are not unusual, but others are. The concentration of CO2 in the atmosphere has reached a record high relative to more than the past half-million years, and has done so at an exceptionally fast rate. Current global temperatures are warmer than they have ever been during at least the past five centuries, probably even for more than a millennium.

If warming continues unabated, the resulting climate change within this century would be extremely unusual in geological terms. Another unusual aspect of recent climate change is its cause: past climate changes were natural in origin, whereas most of the warming of the past 50 years is attributable to human activities.

For more information on this topic, see FAQ 6.2, extracted from Chapter 6 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

How are temperatures on Earth changing?

Instrumental observations over the past 157 years show that temperatures at the surface have risen globally, with important regional variations. For the global average, warming in the last century has occurred in two phases, from the 1910s to the 1940s (0.35°C), and more strongly from the 1970s to the present (0.55°C). An increasing rate of warming has taken place over the last 25 years, and 11 of the 12 warmest years on record have occurred in the past 12 years. Above the surface, global observations since the late 1950s show that the troposphere (up to about 10 km) has warmed at a slightly greater rate than the surface, while the stratosphere (about 10–30 km) has cooled markedly since 1979. This is in accord with physical expectations and most model results.

Confirmation of global warming comes from warming of the oceans, rising sea levels, glaciers melting, sea ice retreating in the Arctic and diminished snow cover in the Northern Hemisphere.

For more information on this topic, see FAQ 3.1, extracted from Chapter 3 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).

How do human activities contribute to climate change and how do they compare with natural influences?

Human activities contribute to climate change by causing changes in Earth’s atmosphere in the amounts of greenhouse gases, aerosols (small particles), and cloudiness. The largest known contribution comes from the burning of fossil fuels, which releases carbon dioxide gas to the atmosphere.

Greenhouse gases and aerosols affect climate by altering incoming solar radiation and out-going infrared (thermal) radiation that are part of Earth’s energy balance. Changing the atmospheric abundance or properties of these gases and particles can lead to a warming or cooling of the climate system.

Since the start of the industrial era (about 1750), the overall effect of human activities on climate has been a warming influence. The human impact on climate during this era greatly exceeds that due to known changes in natural processes, such as solar changes and volcanic eruptions.

Concentrations of Green House Gases in the Atmosphere from the Year 0 to 2005

(Source: Figure 1 FAQ 2.1 Intergovernmental Panel  FAQ 2.1- Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007) 

For more information on this topic, see FAQ 2.1, extracted from Chapter 2 of the Intergovernmental Panel on Climate Change 4th Assessment Report, Working Group 1, The Physicals Science Basis (2007).