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How Do We Know That Global Warming Is Affecting Our World?

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The biggest policy question facing human society today is climate change, and specifically:

How do we know that we really need to act on climate change?

But that's a big question, and so we've broken it down into sub-questions:

1) How do we know that CO2 (and other greenhouse gases) cause planetary warming?

2) How do we know that we are responsible for the measured increase in greenhouse gas concentrations?

3) How do we know that warming will be a bad thing if it continues?

In two previous Hubs, we've addressed the first two questions. If you missed them you can use the links below to jump directly to them.

But granting that CO2 causes warming, and that we are increasing CO2, will the expected warming be, on balance, a bad thing?

This is the hardest question to answer; the story of so-called climate "impacts" is both large and diffuse. We can help a bit by breaking our 'sub-question' down into two 'sub-sub-questions': how do we know that warming affects the ‘real world’, and how do we know that those effects will be on balance bad?

The Preceding Hubs In This Series

Svante Arrhenius, as honored on a Swedish postage stamp celebrating the centenary of his birth.

Svante Arrhenius, as honored on a Swedish postage stamp celebrating the centenary of his birth.

How do we know that warming is affecting things in the ‘real world’?

Let’s look at the history of the study of real-world ‘impacts.’ It begins with logical speculation and extrapolation, continues with more precise calculations of expected impacts based on physics, and moves into the realms of ‘detection’—showing the ‘real world’ existence of impacts—and ‘attribution’—showing that those impacts are, or probably are, due to human influence.

Initially, researchers such Svante Arrhenius and Guy Callendar thought that the impacts of warming would be beneficial. The "deadly ice ages" would be averted, and "more equable" climate would be the result. But as post-World War Two industrial expansion drove sky-rocketing carbon emission, less positive potential impacts kept turning up.

US Navy research vessel RV Roger Revelle, honoring the prominent scientist.

US Navy research vessel RV Roger Revelle, honoring the prominent scientist.

Let's list some notable identifications of fossil fuel or climate change impacts. From the the 1950s to the end of the 1970s:

  • Late 1950s, Scripps Oceanographic Institute Director Roger Revelle: damaging sea level rise, "aridity" possibly creating "real deserts";
  • 1963, Conservation Foundation: dangers to "biological systems";
  • 1965, Presidents' Advisory Council: warming "deleterious" to humans;
  • 1971, Stockholm-based "Study of Man's Influence on Climate" ('SMIC'): Arctic sea ice loss, affecting global circulation and weather;
  • 1974, CIA: risks to food security that could trigger ‘mass migrations, perhaps even wars’;
  • 1977, US National Academy of Sciences: most of above, plus an estimate of possible 4 meter sea level rise within 300 years, plus some benefits such as shorter Arctic shipping routes;
  • 1979, NAS: ocean acidification.
'Sunny day' flooding in Miami Beach.

'Sunny day' flooding in Miami Beach.

The 1980s saw new concerns. The EPA projected in 1983 that by 2100 sea level rise would be causing “major coastal impacts, including shoreline retreat... flooding, saltwater intrusion, and various economic effects.” The same year saw the National Academy pointing out that increasing heat waves might cause “excess human death and illness”, that permafrost melt would likely damage Arctic infrastructure, that risky changes in “disease vectors” could happen, and there could also be unforeseen danger—‘unknown unknowns,’ as we might call them today.

1985 saw the first call to formulate policy responses, when a consortium of international agencies pointed out the potential to create a climate that was literally beyond anything ever experienced by the human species. (A real possibility that remains under-appreciated today: 30 years later, we are actually on track to take our climate into the ‘three degree world’ by the end of the century—a climate that has not been since since the early Pliocene era, more than three million years ago.)

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EPA headquarters, 2006.  Image courtesy Wikimedia Commons, by user Coolcaesar.

EPA headquarters, 2006. Image courtesy Wikimedia Commons, by user Coolcaesar.

A final decadal milestone was reached with a 1989 EPA study that went beyond consideration of mere possibilities, by numerically modeling climate change outcomes—including adaptions to climate change. Since it is unlikely—despite the history of slow and inadequate reaction to climate change so far—that complete inaction in the face of visible change will be the reality, including human responses in the modeling gives a more plausible picture of possible outcomes.

In addition to estimating many of the impacts already mentioned above, the 1989 study identified some new issues as well, notably deforestation and water management problems due to snowmelt and runoff changes—a vivid contemporary example of this is the current California drought, which loss of snowpack has seriously exacerbated.

An IPCC lead authors meeting.  Image courtesy IPCC.

An IPCC lead authors meeting. Image courtesy IPCC.

In the 1990s consideration of possible and observed climate impacts became much more systematic. Rising concern in 1988 prompted the World Meteorological Organization and the United Nations Environmental Program to found a new organization, the International Panel on Climate Change (IPCC). It would be synthesize published research on climate change-related science, and report on the 'state of the art.' Apart from a small administrative staff, its work would be carried out by large teams of volunteer scientists who would be able to bring together encyclopedic knowledge of the most recent research. The result was information of unparalleled scope, assembled and delivered in a highly systematic fashion.

The first comprehensive IPCC "Assessment Report" came out in 1990, with succeeding installments appearing in 1996, 2001, 2007, and 2013. Each report included a volume on the impacts of climate change. Reports of observed impacts were considered, while projected future impacts were examined based on "scenarios"—'story lines' about how society might develop. The ‘story lines’ would lead to assumptions about carbon emissions and so forth that were then used as input to computer modeling studies.

Projected 'emissions pathways', as found in "AR5", the current IPCC Assessment Report.

Projected 'emissions pathways', as found in "AR5", the current IPCC Assessment Report.

FAR cover

FAR cover

FAR, 1990

"FAR" (1990) reported detection of global warming itself, and of sea level rise. But it did not attribute either, yet, to human influence. The authors wrote:

Our judgement is that:

• Global - mean surface air temperature has increased by 0 3°C to 0 6°C over the last 100 years, with the five global-average warmest years being in the 1980s. Over the same period global sea level has increased by 10-20cm. These increases have not been smooth with time, nor uniform over the globe.

• The size of this warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability. Thus the observed increase could be largely due to this natural variability, [or] alternatively this variability and other human factors could have offset a still larger human-induced greenhouse warming. The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more.

SAR cover

SAR cover

SAR, 1995

Despite progress in data collection and scientific analysis, the Second Assessment Report of 1996 did not change this much. Sea level rise estimates had crept up to 10-25 cm and the rise was attributed—or nearly attributed—to warming:

...much of the rise may be related to the increase in global mean temperature.

Some regional-scale effects were detected, as well:

On regional scales there is clear evidence of changes in some extremes and climate variability indicators (e.g., fewer frosts in several widespread areas; an increase in the proportion of rainfall from extreme events over the contiguous states of the USA).

The panel inched toward attributing warming to human influence, writing that:

...the balance of evidence suggests that there is a discernible human influence on global climate.

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TAR cover

TAR, 2001

New detections were featured in the 2001 Third Report. The 1990s had been the warmest decade on record, and:

The global average surface temperature (the average of near surface air temperature over land, and sea surface temperature) has increased since 1861.

Warming of the lower troposphere was noted, too:

Since the late 1950s (the period of adequate observations from weather balloons), the overall global temperature increases in the lowest 8 kilometres of the atmosphere and in surface temperature have been similar at 0.1°C per decade.

Decreasing trends in snow cover, glacier ice mass, and Arctic sea ice cover were all detected. Ocean heat content, on the other hand, was rising.

AR4 cover

AR4 cover

AR 4, 2007

Still less equivocal was the Fourth Assessment Report, of 2007:

The understanding of anthropogenic warming and cooling influences on climate has improved since the TAR, leading to very high confidence that the global average net effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 [+0.6 to +2.4] W m–2...

Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level...

Surface temperature, tropospheric temperature, ocean heat content, glacier ice mass, Arctic sea ice, and sea level rise trends had all continued.

“AR4” also noted many new ‘detections,’ included atmospheric water vapor content (an important climate ‘feedback’ mechanism), ice mass loss in the Greenland and Antarctic ice sheets (as distinct from mountain glaciers and small ice caps), Arctic temperatures, increases in drought, increases in prevailing ‘westerlies’, and increases in both extreme precipitation and extreme warm temperatures and heatwaves.

Changes in permafrost had become quite dramatic:

Temperatures at the top of the permafrost layer have generally increased since the 1980s in the Arctic (by up to 3°C). The maximum area covered by seasonally frozen ground has decreased by about 7% in the Northern Hemisphere since 1900, with a decrease in spring of up to 15%.

More ominously from a food security perspective, regional changes in precipitation were becoming clearer:

Long-term trends from 1900 to 2005 have been observed in precipitation amount over many large regions. Significantly increased precipitation has been observed in eastern parts of North and South America, northern Europe and northern and central Asia. Drying has been observed in the Sahel, the Mediterranean, southern Africa and parts of southern Asia.

Nobel prize medal.

Nobel prize medal.

Many of these trends were also attributed to human influence for the first time in AR4—though the attributions were of varying degrees of firmness, ranging from “virtually certain” down to “more likely than not.”

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. This is an advance since the TAR’s conclusion that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations”. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns...

Pattern studies established a human connection to warming “of surface and atmospheric temperatures in the upper several hundred metres of the ocean, and in contributions to sea level rise,” and showed that:

The observed pattern of tropospheric warming and stratospheric cooling is very likely due to the combined influences of greenhouse gas increases and stratospheric ozone depletion.

(The last is an underappreciated point, since some would still like to argue that global warming ‘must be’ due to the Sun—unlikely, since the upper atmosphere is cooling rather than warming.)

Human influence in observed changes in wind patterns (including extra-tropical storm tracks) and some temperature extremes was considered “likely”, while the increased risk of heat waves was “more likely than not” due to human influence.

AR4 was a landmark not only in its increased certainty and scope, but also in its level of recognition: in 2007, the IPCC won the Nobel Peace Prize for its efforts.

AR5 cover

AR5 cover

AR5, 2014

The newest Assessment Report came out in 2014. Like AR, its hallmarks are increased certainty and scope, with much additional detail. There is far more content than can even be sketched here. The most that can be done is to touch on highlights—beginning with the lead sentences of the Summary: