Global warning: An abridged history of the physical science basis of climate change

[Lisbeth]

By Ethan van Diemen• 5 August 2021


Pakistani boys jump in the water to cool off in a pond to beat the heat wave continues in Larkana, Pakistan, 26 June 2021. (Photo: EPA-EFE/WAQAR HUSSAIN)

Ahead of the release of the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, it is noteworthy that the projections and assertions of increasingly confident climate scientists are happening apace, and that doesn’t bode well for the future unless substantial and sustained changes are made.

“Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems.”

So reads part of the headline statement of the Synthesis Report of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) published in 2014.

Seven years later, in the middle of what the World Meteorological Organization has called a “summer of extremes” in the Northern Hemisphere, and days before the first part of the IPCC’s AR6 is released, many of the projections of the previous reports have seemingly come to fruition.

From severe flooding in parts of Germany, the Netherlands and Belgium, once-in-a-century downpours in China to record-breaking heatwaves over western parts of North America — the evidence is less suppositious and more certain than ever. This relatively new-found certainty is also reflected in the work of the IPCC.

On Monday, 9 August, Working Group 1 of the IPCC will publish its contribution to its Sixth Assessment report, titled AR6 Climate Change 2021: The Physical Science Basis.

Its previous iterations were AR5 Climate Change 2013: The Physical Science Basis and AR4 Climate Change 2007: The Physical Science Basis.

Similar to these reports, AR6 contributes to humanity’s understanding of the physical science behind climate change. Read together with AR5 and AR4, AR6 is expected to be the definitive, most contemporary account of the climate crisis in the 21st century.

To understand how we arrived at this present — buffeted by an increasingly hostile planet — it may be useful to look to the past to see how key indicators have changed and how scientific confidence in attributing these changes in the climate system to human activity has evolved over the years.

Atmosphere

With regard to the atmosphere, the 2007 report noted that “11 of the last 12 years (1995-2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850)”.

The 2013 report expanded on this, saying: “Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.

“It is likely that the frequency of heatwaves has increased in large parts of Europe, Asia and Australia. There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased. The frequency or intensity of heavy precipitation events has likely increased in North America and Europe.”

Given the events taking place across the globe, one would be forgiven for ascribing to scientists an oracular quality.

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Source: Nasa’s Scientific Visualisation Studio

According to an analysis by Nasa, Earth’s global average surface temperature in 2020 tied with 2016 as the warmest year on record.

Ocean

Integral as they are to the global climate system, the AR5 report says of the oceans that envelop most of our blue planet that, “Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971.”

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Source: Nasa’s Scientific Visualisation Studio

The above visualisation shows sea surface temperature data from January 2016 to March 2020.

Cryosphere

Sea ice and the cryosphere are important indicators of the progression of human-caused climate change. The AR4 in 2007 reported that “satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.7 [2.1 to 3.3]% per decade, with larger decreases in summer of 7.4 [5.0 to 9.8]% per decade.”

The AR5, however, updates these figures, explaining that the “annual mean Arctic sea ice extent decreased over the period 1979 to 2012 with a rate that was very likely in the range 3.5 to 4.1% per decade” and “very likely” in the range 9.4 to 13.6% per decade for the summer sea ice minimum (perennial sea ice).

Carbon and other biogeochemical cycles

One of the more prominent indicators of change can be found in the atmospheric concentration of greenhouse gases.

The AR4 Climate Change 2007: The Physical Science Basis report explained that the global atmospheric concentration of carbon dioxide has “increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores.”

“Ppm” or parts per million is the ratio of the number of gas molecules to the total number of molecules of dry air. The natural range over the last 650,000 years, for example, has been 180 to 300 molecules of carbon dioxide per million molecules of dry air.

The AR5 explained, however, that “carbon dioxide concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land-use change emissions”. It continues that in 2011, concentrations of carbon dioxide were 391 ppm, registering an increase of 12 ppm in the years between the two reports.

The latest measurements taken by Nasa in June of this year indicate that carbon dioxide concentration has further increased to 417 ppm.

Carbon dioxide: https://www.dailymaverick.co.za/article ... te-change/
Source: NOAA


Detection and attribution of climate change

A notable change that has taken place in the climate science space is the confidence with which climate change and its associated impacts can be ascribed to post-industrial human activity.

The 2007 report said, “The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to known natural causes alone.”

It also said 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”.

The report that preceded this was less confident, instead saying: “Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.”

By the time AR5 was published in 2013, the language was more direct and assured.

“Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise and in changes in some climate extremes,” reads one paragraph.

“This evidence for human influence has grown since AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

“It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings.”

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Source: Nasa’s Scientific Visualisation Studio

Ahead of the release of the Sixth Assessment Report of the IPCC, it is perhaps noteworthy then that the projections and assertions of increasingly confident climate scientists are happening apace, and that doesn’t bode well for the future unless substantial and sustained changes are made — and soon. DM/OBP

https://www.dailymaverick.co.za/article ... te-change/

[Lisbeth]

Humanity is compressing millions of years of natural change into just a few centuries

November 1, 2021 | Dan Lunt, Professor of Climate Science, University of Bristol - Darrell Kaufman, Professor of Earth and Environmental Sciences, Northern Arizona University

Many numbers are swirling around the climate negotiations at the UN climate summit in Glasgow, COP26. These include global warming targets of 1.5℃ and 2.0℃, recent warming of 1.1℃, remaining CO₂ budget of 400 billion tonnes, or current atmospheric CO₂ of 415 parts per million.

It’s often hard to grasp the significance of these numbers. But the study of ancient climates can give us an appreciation of their scale compared to what has occurred naturally in the past. Our knowledge of ancient climate change also allows scientists to calibrate their models and therefore improve predictions of what the future may hold.


Recent climate changes in context. IPCC AR6, chapter 2

Recent work, summarised in the latest report of the Intergovernmental Panel on Climate Change (IPCC), has allowed scientists to refine their understanding and measurement of past climate changes. These changes are recorded in rocky outcrops, sediments from the ocean floor and lakes, in polar ice sheets, and in other shorter-term archives such as tree rings and corals. As scientists discover more of these archives and get better at using them, we have become increasingly able to compare recent and future climate change with what has happened in the past, and to provide important context to the numbers involved in climate negotiations.

For instance one headline finding in the IPCC report was that global temperature (currently 1.1℃ above a pre-industrial baseline) is higher than at any time in at least the past 120,000 or so years. That’s because the last warm period between ice ages peaked about 125,000 years ago – in contrast to today, warmth at that time was driven not by CO₂, but by changes in Earth’s orbit and spin axis. Another finding regards the rate of current warming, which is faster than at any time in the past 2,000 years – and probably much longer.

But it is not only past temperature that can be reconstructed from the geological record. For instance, tiny gas bubbles trapped in Antarctic ice can record atmospheric CO₂ concentrations back to 800,000 years ago. Beyond that, scientists can turn to microscopic fossils preserved in seabed sediments. These properties (such as the types of elements that make up the fossil shells) are related to how much CO₂ was in the ocean when the fossilised organisms were alive, which itself is related to how much was in the atmosphere. As we get better at using these “proxies” for atmospheric CO₂, recent work has shown that the current atmospheric CO₂ concentration of around 415 parts per million (compared to 280 ppm prior to industrialisation in the early 1800s), is greater than at any time in at least the past 2 million years.


An IPCC graphic showing climate changes at various points since 56 million years ago. Note most rows show changes over thousands or millions of years, while the top row (recent changes) is just a few decades. IPCC AR6, chapter 2 (modified by Darrell Kaufman)

Other climate variables can also be compared to past changes. These include the greenhouse gases methane and nitrous oxide (now greater than at any time in at least 800,000 years), late summer Arctic sea ice area (smaller than at any time in at least the past 1,000 years), glacier retreat (unprecedented in at least 2,000 years) sea level (rising faster than at any point in at least 3,000 years), and ocean acidity (unusually acidic compared to the past 2 million years).

In addition, changes predicted by climate models can be compared to the past. For instance an “intermediate” amount of emissions will likely lead to global warming of between 2.3°C and 4.6°C by the year 2300, which is similar to the mid-Pliocene warm period of about 3.2 million years ago. Extremely high emissions would lead to warming of somewhere between 6.6°C and 14.1°C, which just overlaps with the warmest period since the demise of the dinosaurs – the “Paleocene-Eocene Thermal Maximum” kicked off by massive volcanic eruptions about 55 million years ago. As such, humanity is currently on the path to compressing millions of years of temperature change into just a couple of centuries.


Many mammals, like these horse-ancestors ‘Eohippus’, first appeared after a sudden warm period 55 million years ago. Daniel Eskridge / shutterstock

Distant past can help predict the near future

For the first time in an IPCC report, the latest report uses ancient time periods to refine projections of climate change. In previous IPCC reports, future projections have been produced simply by averaging results from all climate models, and using their spread as a measure of uncertainty. But for this new report, temperature and rainfall and sea level projections relied more heavily on those models that did the best job of simulating known climate changes.

Part of this process was based on each individual model’s “climate sensitivity” – the amount it warms when atmospheric CO₂ is doubled. The “correct” value (and uncertainty range) of sensitivity is known from a number of different lines of evidence, one of which comes from certain times in the ancient past when global temperature changes were driven by natural changes in CO₂, caused for example by volcanic eruptions or change in the amount of carbon removed from the atmosphere as rocks are eroded away. Combining estimates of ancient CO₂ and temperature therefore allows scientists to estimate the “correct” value of climate sensitivity, and so refine their future projections by relying more heavily on those models with more accurate climate sensitivities.

Overall, past climates show us that recent changes across all aspects of the Earth system are unprecedented in at least thousands of years. Unless emissions are reduced rapidly and dramatically, global warming will reach a level that has not been seen for millions of years. Let’s hope those attending COP26 are listening to messages from the past.