Science of Climate Change

FREQUENTLY ASKED QUESTIONS ABOUT THE SCIENCE OF CLIMATE CHANGE

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C. Detecting and Attributing Climate Change

C.1 Has the world warmed?

Response: Yes. The average global temperature at the Earth's surface has warmed by about 0.6°C since the late 19th century.

[View] (D)

Explanation: Best estimate for the magnitude of average global surface warming over the past century is 0.6°C, with an error range of +/-0.2°C . It is important to note that these refer to average global surface warming. In some areas, particularly over continents, the warming has been several times greater than the global average. In a few areas, temperatures have actually cooled. In Canada, for example, there has been an increase in average annual temperature of about 1°C over the period 1895-1992.  According to recent research results for the Northern Hemisphere, the 20th century is now likely the warmest century, the 1990s the warmest decade, and 1998 and 2001 the hottest years of the past millennium.

C.2 How do scientists know that the Earth has warmed?

Response: In addition to the consistency of evidence for warming obtained from both the instrumental air temperature records and other proxy temperature data, there are many other indicators of a warming world. These include warming of the upper layers of the world's oceans, melting mountain glaciers, retreating sea ice and snow cover, rising sea levels, and shifts in distribution of many species of plants and animals.

Explanation: The instrumental records of surface atmospheric temperatures collected over the past 120 years agrees well with proxy indicators of climate, such as data from tree ring, ice cores, corals and ground temperatures. All show substantial warming over the past century. Furthermore, as shown in Figure C.1, when these proxy records are extended back, they show that the 20^th century warming in the Northern Hemisphere is unprecedented in at least the past 1000 years, and that the 1990s was the warmest decade and 1998 and 2001 the two single warmest years for that time period. Furthermore, a variety of other climate variables provide supporting evidence of a warming climate. These include: a 10% reduction in the extent of snow cover in the Northern Hemisphere since the late 1960s; a coincident reduction in Northern Hemisphere lake ice cover seasons; a reduction in Arctic sea ice cover since the 1950s of 10 to 15% and a considerable decline in sea ice thickness; a 10 to 20 cm global sea level rise during the 20^th century; an increase in global ocean heat content since adequate measurements began in the 1950s; and an decrease in the frequency of extreme low temperatures since the 1950s.

C.3 Despite the overall global warming during the 20th century, some argue that current average temperatures are still lower than during warm periods experienced in the past, such as the Medieval Warm Period.  Doesn't this suggest that current increases are likely due to natural causes, and therefore of no real concern?

Response:  Natural causes, such as increased sunlight intensity and reduced volcanic dust in the atmosphere, may have contributed to the warming in the first past of the 20th century, but cannot explain the rapid warming during the most recent half-century. However, Figure C.1 shows that the warming in recent decades is consistent with that expected due to human interference with the atmosphere. Furthermore, as illustrated in Figure C.3, careful analysis of both measured and indirect indicators of global temperatures indicate that the 20th century was very likely the warmest century of at least the past millennium, and the 1990s were likely the warmest decade of this period. The IPCC concludes that most of the warming during the past 50 years was likely due to human influences. 

[View] (D)

Explanation: Researchers have indirectly collected information about past climates from various indicators such as tree rings, ice cores and ocean corals. These indicate that, for at least the Northern Hemisphere, the 20^th century was the warmest in at least the past 1000 years. Furthermore, the 1990s was the warmest single decade. By comparison, the Medieval Warm Period of about 1000 years ago appears to have been warm in regions surrounding the North Atlantic but not in other parts of the Northern Hemisphere. Hence average temperatures for the entire hemisphere during that period were cooler than that for the past century (see Figure C.1). Proxy data for the Southern Hemisphere are as yet too sparse to make similar conclusive comparisons in that region. However, paleo-climate scientists have also made some approximations of global temperatures further back in time. These suggest that temperatures experienced during the peak of the current interglacial period some 6-8,000 years ago were about 1°C warmer than today, and that temperature variations within this range have occurred on thousand year time scales since then.  This suggests that some of the recent warming could be due to natural causes.   As shown in Figure C.3, climate model studies indicate that, during the first half of the 20^th century, a significant part of the warming is, in fact, likely due to a combination of increased solar radiation, decreased volcanic dust in the atmosphere, and rising greenhouse gas concentrations. However, during the past 50 years, solar intensity has not shown a significant long-term trend and more frequent major volcanic eruptions have, on average, increased the level of volcanic dust in the atmosphere with time. Thus the combined effects of the natural causes for change, by themselves, would have caused cooling during that period. In contrast, the observed climate record shows a rapid warming in recent decades consistent with that expected due to human influences. Hence, the IPCC concludes that, while the changes during the past century is due to a combination of natural and human factors, that for the past 50 years is likely due to the dominance of human influences.

References: IPCC, 2001 WGI, Chapter 2 and 12.

C.4 Since temperature records of the past century may be distorted by observational errors, relocation of observing sites, and other human influences such as the urban heat island effect, can we rely on them to determine how the climate is changing?

Response:  Yes, collectively they provide a good indication of how our climate is changing. As required for proper use of data from all monitoring programs, the climate data used to study the climate is first evaluated for quality and systematic sources of error. In addition to deleting records with major errors or non-climatic influences and correcting others where the error is readily identifiable, climate scientists also compare the climate records used with other types of information.  To allow for any remaining non-climate factors affecting these records, experts provide a margin of error in their estimates.  They state with confidence that the warming over the past century has been at least 0.4°C, and not more than 0.8°C.

[View] (D)

Explanation: One method of dealing with random errors that occur at single stations is to average the temperature values over many stations. Global temperature analyses use many thousands of stations, and hence such random errors are largely removed through averaging. Systematic changes that are un-related to climate but that can affect many or all of the records at the same time or in the same way are more difficult to remove. These include changes in observed values due to urban heat island effects, large-scale changes in instrumentation, changes in the density of recording stations, or a systematic shift in the location of instruments at weather stations. These can be at least partially addressed through careful analysis and adjustments.  In undertaking the global trend analyses, climate experts have made careful allowance for a number of such systematic influences, including the heat island effect, the change in observing processes on ships, and other non-climatic influences on observations.  There remains solid evidence that the warming of the recent decades is real and global.  Furthermore, surface temperature records are in good agreement with the long term trend apparent in upper air radiosonde measurements over the past half-century, with evidence from tree rings, and with information obtained from bore holes drilled into the earth's surface in different parts of the world.  They are also consistent with concurrent trends towards reduced global snow cover, glacier retreats and other indicators of a warming world.  However, because of the uneven global distribution of observation sites, climate records are still dominated by land data obtained in the Northern Hemisphere. Considering these uncertainties, the science community estimates that the earth's surface has, on average, warmed by at least 0.4°C and possibly by as much as 0.8°C.

C.5 A large increase in temperature occurred in the early part of this century when CO₂ emissions were still relatively low.  However, temperatures actually cooled in the 1950s and 1960s, when emissions began to increase rapidly. Doesn't this contradict the idea that increased CO₂ emissions will cause warmer climates?

Response: In addition to the effects of CO₂, the temperatures of the past century have been influenced by changes in the climate system due to natural factors (such as internal variability within the climate system, solar variability and/or emissions of aerosols due to volcanic eruptions), by increased concentrations of other pollutants released into the atmosphere by human activities (such as sulphate aerosols and dust), and most recently by ozone depletion in the stratosphere.  The response to these various causes of change is further modified by the slow response of oceans.  When all these factors are included in model simulations of temperature change, the results agree quite closely with those observed.

Explanation: The behaviour of the climate system is complex, involving many different components, variables and feedbacks.  The system has significant natural internal variability that causes fluctuations in climate conditions from one region to the next and one decade to the next.  For example, fluctuations associated with the El Nino-Southern Oscillation and the North Atlantic Oscillation can alter global temperature patterns for years and even decades.  In addition, the climate system responds to a variety of different causes of change (called climate forcings).   Of these, an increase in greenhouse gas concentrations, although probably dominant during the past century, is only one.  These include the effects of natural changes in solar intensity (higher intensity means warmer surface temperatures) or the amount of volcanic dust in the atmosphere (which tends to cool the surface), as well as other human influences such as emissions into the atmosphere of various types of aerosols or their precursors (primarily a surface cooling effect), and stratospheric ozone depletion (a surface cooling effect). These effects vary in time. The cooling offset from human emissions of sulphate aerosols, for example, increased rapidly in the Northern Hemisphere between the 1950s and the 1980s, thus masking some of the influence of greenhouse gas increases in recent decades, but has stabilized in recent decades because of measures to improve air quality.  In contrast, the surface cooling influence of ozone depletion began in the late 1970s, and is now peaking as the measures of the Montreal protocol to reduce emissions of ozone depleting substances take effect.

Furthermore, the long-term response of climate to the effects of increases in CO₂ concentrations or climate forcing is delayed by a significant lag effect due to ocean inertia, much like large inland lakes can influence and delay the seasonal changes in adjacent land temperatures.   The more rapid the increase in the cause for climate change is, the greater the gap between the instantaneous response of the climate and the full potential of its response to the forcing becomes.  Thus, because the rate of increase in CO₂ up to 1940 was quite slow (over 150 years), most of the related effects on climate would already have been fully evident within the climate system by 1940.  However, since the increase in CO₂ since 1940 has been much more rapid, ocean inertia will have delayed a much larger fraction of its full potential climatic effect. 

The upward trend in temperature observed over the past century is dominated by a fairly abrupt warming of about 0.3°C between 1920 and 1940, a slight cooling trend between 1940s and 1970s, and another period of rapid warming (about 0.3°C) between 1975 and today.  When the various forcing factors mentioned above are all included in model simulations, the projections agree quite well with the observed pattern of change.  They also suggest that the human factors have dominated the trends during the past 50 years.

Reference: IPCC 2001, WGI, Chapter 12.

C.6  Doesn't the substantial cooling in places like the eastern Canadian Arctic, Greenland and in eastern Antarctica over the past few decades contradict model predictions of global warming?

Response: No. A regional cooling can be fully consistent with a warmer world. Although increasing greenhouse gas concentrations apply a rather uniform global forcing, other factors such as natural variability, local feedbacks and regional changes in atmospheric and oceanic circulation can enhance their effects in region while reducing them in another. For example, in the Arctic, some regions, such as the western Canadian Arctic and Siberia, have warmed dramatically. Although the eastern Arctic also warmed slightly during the past 50 years, some regions within it have cooled somewhat. Despite these regional variations, the average temperatures across the Arctic are becoming considerably warmer in a manner largely consistent with recent model projections.  Likewise, while some regions of east Antarctica have cooled over the past few decades, the Antarctic Peninsula has warmed dramatically.  Advanced climate models are now able to capture this variability quite well, and can simulate regional changes that are broadly similar to that observed.

Explanation: The climate system is highly variable, both from region to region and in time.  This is because local feedbacks, changes in atmospheric circulation and other factors can affect one part of a region much differently than another. For example, there is evidence that, over the past 50 years, changes in the Arctic/North Atlantic Oscillations have been important factors in causing the western Canadian Arctic and parts of Siberia to warm dramatically (by more than 2°C) while parts of the eastern have cooled.  Despite the areas of cooling, the eastern Arctic has still warmed slightly (about 0.2°C). Likewise, in Antarctica, parts of the eastern region of the continent have cooled in recent decades, while the Antarctic Peninsula has warmed significantly. In this region, there is evidence that regional atmospheric circulation changes that contributed to these regional differences may be linked to stratospheric ozone depletion. Some of these differences also disappear when the length of the record is increased. For example, while the entire Atlantic Canada region has cooled slightly during the past 50 years, the Maritime Provinces show a significant warming of 0.6°C when the full 100 year record available for the region is considered. Meanwhile, supporting evidence for large scale warming trends in Arctic regions include a decrease of some 400,000 square kilometers in the extent of sea ice and a concurrent decrease in snow cover in the Northern Hemisphere over the past decade.

Climate simulations with coupled climate models are as yet not able to predict the precise decade-to-decade shifts in these regional changes in atmospheric circulation and other factors affecting regional temperatures. However, they do show regional patterns of change that show differences broadly similar to that observed.

C.7 Satellite measurements show temperatures in the lower atmosphere above the earth's surface are warming much more slowly than the surface.  Doesn't this imply that the earth isn't warming as expected?

Response: No. When the surface data is compared with lower atmospheric temperatures obtained from weather balloon data for an extended period of more than four decades, the two trends are almost identical.   Unfortunately, the satellite data is only available for the past two decades. On such short time scales, unusual climate events such as volcanic eruptions and El Niños can significantly bias trends, and affect the surface differently than the atmosphere above it. Hence, the satellite data record by itself is still too short to be useful for long-term climate tends analysis.

[View] (D)

Explanation: Microwave satellite data have been used since 1979 to estimate temperatures of the lower troposphere between about 8 km altitude and the surface.  Over this short period of time, the trends observed at the surface and in the lower atmosphere are both significantly influenced by the warming influence of a strong El Niño event in 1982-83 and the cooling influence of increased volcanic dust in the atmosphere for several years following the eruption of Mt. Pinatubo in 1991.  However, these influences affect the surface differently than the lower atmosphere. Much of these differences should average out over time. In fact, comparison of mean surface temperature trends with those for the lower atmosphere compiled from radiosonde data suggest the two trends are almost identical when averaged over the past four decades. 

Like surface data records, the satellite data must also be corrected for changes unrelated to the climate. For example, while earlier estimates of trends in the satellite data suggested a cooling trend of about -0.06°C per decade since 1979, recent corrections for the influence of satellite orbital decay and the addition of several more years of data have resulted in a slight warming trend of 0.04°C per decade.   There are also concerns about other possible inaccuracies in the data, since the record consists of measurements taken by instruments on eight different satellites and pieced together into a single series. Furthermore, the effects of water vapour and droplets in the atmosphere on the microwave data may not have been adequately corrected when calculating the related temperatures. 

Reference: IPCC 2001, WGI, Chapter 2.

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