- Flora Whitmarsh, science communications research analyst1,
- Brian Hoskins, professor1,
- David McCoy, senior clinical lecturer and director2
- 1Grantham Institute for Climate Change, Imperial College London, London, UK
- 2Department of Primary Care and Public Health, Queen Mary University London, London, UK
- Correspondence to: D McCoy
After The BMJ published an article on the science of anthropogenic climate change,1 it received questions about various aspects of climate science. Here we answer some of those questions. Like the original article, we draw on the findings of the fifth report of the UN commissioned Intergovernmental Panel on Climate Change (IPCC).2 The main body of the report is written by working scientists, drawing on the findings of thousands of peer reviewed scientific studies, and is the most comprehensive analysis of climate science available. Last year, the Royal Society and the US National Academy of Sciences also published a report, Climate Change: Evidence and Causes, which summarises the status of climate change science.3
How is the reported increase in Antarctic sea ice compatible with global warming?
Sea ice is not to be confused with the land based Greenland and Antarctic ice sheets, both of which are shrinking, as expected with rising temperatures. There has also been an overall global decline in mountain glaciers, permafrost, and snow cover, which is consistent with anthropogenic climate change. Whereas the extent of Arctic sea ice has decreased by 0.73-1.07 million km2 per decade since 1979, Antarctic sea ice has increased by 0.13-0.20 million km2 per decade.2
The anomaly in the Antarctic is due to its geography. Unlike in the Arctic, where the extent of sea ice is constrained by the North American and Eurasian land masses, Antarctic sea ice forms in the open ocean with less land constraining its formation. Antarctic sea ice is also thinner and mostly melts each summer, whereas Arctic sea ice survives longer (although the amount of sea ice lasting more than two years has declined rapidly since 1979). Changing patterns of Antarctic sea ice drift may also be linked to changes in local winds4 that may have been caused by the stratospheric ozone hole in the southern hemisphere.5
Although the surface area of sea ice has increased, the change in the volume of Antarctic sea ice is uncertain because of a lack of data.
Half the temperature increase is claimed to have happened before 1950 while most CO2 emissions have taken place since then. Doesn’t this suggest that CO2 emissions are not responsible for global warming?
This claim is incorrect. In the IPCC reports, linear trends for surface temperature (which are based on three datasets) show that global average surface temperatures increased by 0.85°C between 1880 and 2012, and by 0.72°C between 1951 and 2012. The evidence is strong that most of the warming since 1880 took place after 1950.
Some media reports say that the climate is less sensitive to CO2 than the IPCC previously thought. Have predictions of global warming been exaggerated and can we have faith in the IPCC’s future climate change projection models?
The measure of long term warming, which takes into account feedbacks acting over the course of several centuries, was estimated in the recent IPCC report to be between 1.5°C and 4.5°C. In the previous IPCC report, published in 2007, the lower end of this range was 2°C. The decision to reduce the lower bound (from 2°C to 1.5°C ) in the more recent report formed the basis for claims in the media that the climate is less sensitive to CO2 than previously thought.
However, short term warming due to a doubling of CO2 over 70 years was estimated to be between 1°C and 2.5°C in the latest IPCC report, which is roughly in agreement with the 2007 estimate. This short term warming estimate is more relevant for policy because it defines the warming we might expect this century.
A more recent study, based on inserting observed values in a very simple model, estimated the short term warming to be 1.05-1.80°C.6 Although the upper end of this estimate is lower than that of the IPCC report, the study’s findings still mean that it would only increase the time taken to reach a given level of warming by a few years. Thus, while there are uncertainties about the precise relation between increases in the rate and concentration of atmospheric CO2 with the rate of global warming, these uncertainties have relatively little bearing on what we need to do in terms of climate policy.
Why is rising sea level a problem when this seems to be so slow at the moment?
Global sea level rose by an average of 19 cm between 1901 and 2010, at a rate of 1.7 mm/year. However, the rate of rise has been increasing and is currently about 3 mm/year and projected to be 8-16 mm/year by 2081-2100. This would result in global sea levels 26-82 cm higher than the 1986-2005 average, depending on emissions. Under the IPCC’s highest emissions scenario, we are likely to see changes of up to a metre.
According to the IPCC, the incidence and magnitude of the peak sea level rises have probably already increased since 1970, mainly as a result of average sea level rise. This trend is expected to continue, with an increased risk of sea defences being topped during peak sea levels. An estimated 1-2.3% of the global population live within 1 metre of sea level.7 Continued sea level rises compounded by increases in storm surges and intensity of storms and coastal erosion will place these and many more people at severe risk.
Is it true that the stratosphere is cooling, and how can this be compatible with global warming?
The stratosphere (the portion of the atmosphere between 10-18 km and 50 km above the land) has indeed cooled over the past 50 years, although this cooling has levelled off recently. This is because, with increased greenhouse gas content, the stratosphere loses heat more effectively to space, as well as other factors such as ozone depletion and declining water vapour levels in the stratosphere. However, this does not negate the fact that ocean, land, and atmospheric temperatures below the stratosphere are increasing because of the greenhouse effect.
How can global warming be consistent with increasing cold weather in some locations?
Although global average surface temperatures have been increasing over decades because of human induced climate change, temperatures also fluctuate over shorter timescales because of fluctuations in the amount of solar radiation reaching the Earth’s surface and internal climate variability.
As well as natural fluctuations in solar activity, the amount of solar radiation reaching the Earth’s surface is affected by volcanic eruptions emitting small particles into the atmosphere. These reflect solar radiation and cause the globe to cool for one or two years.
Natural internal climate variability can also affect global surface temperatures through phenomena such as El Niño, which causes a release of heat from the ocean to the atmosphere, and La Niña, which causes the ocean to take up heat. Atmospheric and surface temperatures can also be strongly affected by heat exchange across the ocean. For example, in higher latitudes such as near the UK, the weather is influenced by shifts in the jet stream, which can lead to localised warm or cold spells depending on the time of year and the weather pattern.
Is the claim that there hasn’t been any significant rise in temperatures over the past 18 years true, and can this be compatible with global warming?
Global average surface temperatures have definitely increased over the past 150 years. However, there has been substantial variability, including periods of short term cooling. The rate of increase in global temperature has been much smaller since 1998 compared with the previous 50 years. The is due to a combination of a downward phase of the solar cycle reducing the amount of solar radiation reaching the Earth, a series of small volcanic eruptions, and a redistribution of heat from the surface to the deeper layers of the ocean.
The absorption and distribution of heat within the ocean is especially important. Around 90% of the excess energy caused by increasing greenhouse gases is stored in the oceans, most of which is in the top layers of the ocean (above 700 m of depth). Heat penetrates to the lower layers of the ocean only when it is carried there by ocean currents.
Heating of the shallower layers is not uniform either: the El Niño Southern Oscillation affects the uptake of heat in the surface layers of the Pacific from year to year, and similar processes occurring on decadal timescales may have contributed to the slower rate of global warming since 1998. However, this “pause” in temperature rise is inevitably coming to an end. Physics dictates that greenhouse gases will result in an increase in energy in the Earth-atmosphere system, which will result in an increase in temperature.
Isn’t the natural interglacial cycle of the Earth more important than any human influence on the climate? And won’t global warming help prevent the Earth from entering a new ice age?
The current interglacial era, which has lasted since the end of the last ice age, is driven primarily by changes in the Earth’s orbit around the Sun; this varies on timescales of tens to hundreds of thousand years.
Changes in the distance of the Earth from the Sun, caused by the Earth’s elliptical orbit, influence the amount of solar radiation reaching the Earth. Changes in the tilt (wobble) of the Earth’s axis of rotation also affect the amount of solar radiation that reaches the high latitudes in summer, which is critical to the onset or retreat of glaciation. Furthermore, interglacial warm periods are enhanced by feedbacks such as the release of greenhouse gases caused by the initial warming.
The current warming due to greenhouse gas emissions is occurring much more quickly than warming would occur in the interglacial cycle and is happening during a warm period in that cycle. Concern over what we are doing to the climate should outweigh any concern over a move to a new ice age. Indeed, scientists now refer to the Earth being in the Anthropocene, a geological period in which human effects dominate over natural processes.
But didn’t we also have a Little Ice Age between 1400 and 1700?
The Little Ice Age was particularly apparent in the European region but didn’t represent a major decline in temperatures globally. It was probably caused by a decline in solar output in the second half of the 17th century (known as the Maunder minimum) and a higher level of volcanic activity. These factors cannot account for the long term global warming observed since industrialisation because there has not been a long term increase in solar output or a decline in volcanic activity of the necessary magnitude.
Wasn’t the Earth even warmer in the Pliocene epoch before humans existed?
The Pliocene epoch, which occurred 4 to 5 million years ago, was about 2°C to 3°C warmer than today even though atmospheric carbon dioxide levels are believed to have been similar to today’s levels. Temperatures were higher because of feedbacks in the climate system that occur on timescales much longer than a century. For example, ice sheet loss greatly increases the absorption of solar radiation, leading to overall warming of the planet. This process takes millennia.
Isn’t the IPCC a political body? Can we trust it to represent the science of climate change correctly and reliably?
The IPCC was established by governments through the UN system. However, there are three levels of government acceptance for the IPCC’s reports. The executive summaries of each of the three reports (on climate science, adaptation, and mitigation) have to be approved by the UN governments. This involves line by line discussion between governments and the scientists who wrote the reports. The IPCC synthesis report, which is a summary of the three individual reports, is adopted by UN governments after a discussion of the content, section by section. But the technical summary and individual chapters of each of the three IPCC reports are accepted by governments without any detailed governmental discussion.
This process has led to claims that the IPCC executive summaries are essentially political documents. However, this is not the case: each statement in the executive summaries is linked to statements in the main body of the relevant report, which in turn is written by hundreds of scientists who are unlikely to agree to any changes to the executive summaries that were not consistent with what they had written. The approval of the executive summaries by world governments lends political weight to the IPCC reports, but they remain evidence based scientific documents.
Cite this as: BMJ 2015;351:h6216
Contributors and sources: FW holds a PhD in physical oceanography from Imperial College London and has extensive experience of writing on the science of climate change for a variety of audiences. She has written a briefing paper on ocean heat uptake and its relationship to the global warming pause and contributed evidence to a parliamentary inquiry into the IPCC's fifth assessment report. BH is a climate scientist who was the first director of the Grantham Institute for Climate Change at Imperial College London. He was vice-chair of the Joint Scientific Committee for the World Climate Research Programme, president of the International Association of Meteorology and Atmospheric Sciences, and a member of the science academies of the UK, USA, China, and Europe. DMcC is a medical doctor and public health specialist. He was a member of the first Lancet-UCL Commission on Climate Change and Health and currently studies and teaches on a variety of ecological public health topics.
Competing interests: We have read and understood BMJ policy on declaration of interests and declare BH is a member of the IPCC’s Working Group 1 and DM is director of Medact, a registered charity that promotes sustainable and equitable development.
Provenance and peer review: Commissioned; not externally peer reviewed.
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