Unique and unnatural: modern warming from an historical viewpoint

A Roman altar with the Sun in its chariot on the left, and Vulcan, the god of fire and volcanoes on the right. The climate gods long favoured the Roman Empire, with wobbles in Earth's orbit credited for increasing the amount of solar energy falling on Earth at the time. Image copyright: Nick Thompson, used via Flickr Creative Commons License.

A Roman altar with the Sun in its chariot on the left, and Vulcan, the god of fire and volcanoes on the right. The climate gods long favoured the Roman Empire, with Earth’s orbital dance credited for increasing the amount of solar energy falling on Earth at the time. Image copyright: Nick Thompson, used via Flickr Creative Commons License.

Our climate has changed before. It’s something most of us realise and can agree on and, according to Skeptical Science, it’s currently the most used argument against human-caused warming. If such changes have happened naturally before, the argument goes, then surely today’s warming must also be natural. It’s an appealing idea, with an instinctively ‘right’ feel. Nature is so huge compared to us puny humans, how can we alter its course? The warming we’re measuring today must just be a natural fluctuation.

It’s such an appealing argument that at the beginning of the 20th century that’s just what many scientists thought – that humans couldn’t alter Earth’s climate. In the time since, our knowledge has come a long way. We’ve explored space, become able to build the electronics that are letting you read this, and climate science has likewise advanced and benefited from these advances.

So what do we know today that might convince the sceptical scientists of 115 years ago that we’re warming the planet? Recently, Richard Mallett, one of my Twitter friends who describes himself as sceptical about mainstream climate science, made a point that serves as an excellent test of our current knowledge:

Of the historical warmings he’s referring to, perhaps the least familiar is the Holocene, which is ironic, as the Holocene is now. It’s the current period of geological time that started at the end of the last ice age, 11,700 years ago. By 1900 scientists would have known the term, but they couldn’t explain why it wasn’t as icey as before.

Three variables of the Earth’s orbit—eccentricity, obliquity, and precession—affect global climate. Changes in eccentricity (the amount the orbit diverges from a perfect circle) vary the distance of Earth from the Sun. Changes in obliquity (tilt of Earth’s axis) vary the strength of the seasons. Precession (wobble in Earth’s axis) varies the timing of the seasons. For more complete descriptions, read Milutin Milankovitch: Orbital Variations Image credit: NASA/Robert Simmon.

Three variables of the Earth’s orbit—eccentricity, obliquity, and precession—affect global climate. Changes in eccentricity (the amount the orbit diverges from a perfect circle) vary the distance of Earth from the Sun. Changes in obliquity (tilt of Earth’s axis) vary the strength of the seasons. Precession (wobble in Earth’s axis) varies the timing of the seasons. For more complete descriptions, read Milutin Milankovitch: Orbital Variations. Image credit: NASA/Robert Simmon.

The explanation we have today comes thanks to the calculations Milutin Milanković worked out by hand between 1909 and 1941. Milutin showed that thanks to the gravitational pull of the Moon, Jupiter and Saturn, Earth’s orbit around the Sun varies in three ways. Over a cycle of roughly 96,000 years our path varies between more circular and more oval shapes. The other two ways come because Earth’s poles are slightly tilted relative to the Sun’s axis, which is why we have seasons. The angle of that tilt shifts over a roughly 41,000 year cycle. Earth also revolves around that tilted axis, like a spinning top does when it slows down, every 23,000 years.

Together these three cycles change how much of the Sun’s energy falls on and warms the Earth, in regular repeating patterns. Though that idea would be the subject of much controversy, by the 1960s data measured from cylinders of ancient ice and mud would resolve any doubt. The slow descent into ice ages and more abrupt warmings out of them – like the one that ushered in the Holocene – come from Earth’s shimmies in space. Read the rest of this entry »

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The witness who collided with government on climate

  • This is part two of this profile. Read part one here.
Jim Hansen giving testimony at a US Congressional hearing in 1988, where he'd declare 99% certainty that humans are changing the climate. Image credit: NASA

Jim Hansen giving testimony at a US Congressional hearing in 1988, where he’d declare 99% certainty that humans are changing the climate. Image credit: NASA

“It’s time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here.” It’s a comment that wouldn’t sound out of place today, but Jim Hansen made it 26 years ago, on June 23, 1988, amid record 38°C temperatures in Washington DC. Jim said it to reporters after telling a Congressional hearing he was 99% certain the world is getting warmer thanks to human-made greenhouse gases.

Jim’s 1980s media bombshells led journalist Robert Pool to liken him to a religious ‘witness’, ‘someone who believes he has information so important that he cannot keep silent’. However, he still felt shy and awkward, preferring to immerse himself in pure science, and so would turn down almost all invitations to speak out for another decade. Jim’s efforts during that period would then help build even stronger evidence on global warming. But with extra motivation provided by clashes with the US government and the arrival of his grandchildren he would return to bear witness more forcefully than ever.

Before his self-imposed media ban Jim would make headlines one more time in 1989, after giving written evidence to a hearing convened by then US senator Al Gore. The testimony reaching the hearing had been altered by the White House to make his conclusions about the dangers of global warming seem less certain. When Jim sent the future vice-president a note telling him this, he alerted the media, turning their scheming into the lead story across all TV networks that evening. John Sununu, aide to then president George H. W. Bush, would then try to get Jim fired for his troubles. But Republican senator John Heinz intervened on Jim’s behalf, and he kept his job.

The reputation Jim had built up as a warming witness went ahead of him in December 1989, as he walked into a ‘roundtable’ meeting held by senators Al Gore and Barbara Mikulski. On the coldest day of the year, in a building whose heating system had failed, Al noticed Jim enter and said, “Hey, aren’t you the guy who…” Despite such jibes, Jim was becoming firmer in his convictions. In April 1990 he offered a group of climatologists an even money bet that one of the next three years would be the warmest in a century. He’d be proven right by the end of the year. Read the rest of this entry »

When the climate change fight got ugly

  • Steve Schneider talks about climate and energy with Johnny Carson on the Tonight Show in 1977, early on in his efforts to bring human-caused climate change to the public's notice.

    Steve Schneider talks about climate and energy with Johnny Carson on the Tonight Show in 1977, early on in his efforts to bring human-caused climate change to the public’s notice.

    This is part two of this profile. Read part one here.

“How many of you think the world is cooling?” That’s what Steve Schneider asked the studio audience of the Tonight Show with Johnny Carson in September 1977. And when the majority put their hands up, he explained that the recent cooling trend had only been short-term. Though the unscripted poll meant Steve wasn’t invited back to the programme, through the summer of that year he had brought climate science to US national TV. The appearances typified Steve’s efforts to bring climate change to the world’s notice – efforts that would later draw attention of a less desirable sort.

Building on his earlier high profile research, Steve had just published ‘The Genesis Strategy: Climate and Global Survival’, predicting ‘demonstrable climate change’ by the end of the century. Whether human pollution would cause warming or cooling, he argued governments should copy the biblical story where Joseph told Pharoah to prepare for lean years ahead. In a decade already torn by rocketing food and oil prices, the advice resonated with many who wanted to head off any further crises.

Some scientists criticised Steve and those like him for speaking straight to the public. In particular, climate science uncertainties were so great that they feared confusion – like that over whether temperatures were rising or falling – was inevitable. That dispute grew from a basic question about science’s place in society. Should researchers concentrate on questions they can comfortably answer using their existing methods? Or should they tackle questions the world needs answered, even if the results that follow are less definite?

At a meeting to discuss climate and modelling research within the Global Atmospheric Research Programme (GARP) in 1974 near Stockholm, Sweden, Steve pushed for the second approach. Given the food problems the world was struggling with at the time, it seemed obvious that climate change impacts like droughts, floods and extreme temperatures would bring famines. “I stood alone in arguing that we had to consider the implications of what we were researching,” Steve later wrote. While some attacked him angrily, saying they weren’t ready to address these problems, conference organiser Bert Bolin agreed that socially important questions must be answered.

The suggestion was also controversial because it meant blurring the lines between climate science and other subjects, such as agriculture, ecology and even economics. The director at the US National Center for Atmospheric Research (NCAR) in Boulder, Colorado, where Steve worked, warned that crossing subject boundaries might cost him promotion. But he responded with characteristic wilfulness, founding a journal doing exactly what he was warned not to. Read the rest of this entry »

Enhanced fingerprinting strengthens evidence for human warming role

Microwave sounding units, like the AMSU units on the Aqua satellite, shown here, can be used to take temperature measurements from different layers in the atmosphere. Ben Santer and his colleagues use this information to find a 'fingerprint' of human impact on recent climate changes. Credit: NASA

Microwave sounding units, like the AMSU units on the Aqua satellite, shown here, can be used to take temperature measurements from different layers in the atmosphere. Ben Santer and his colleagues use this information to find a ‘fingerprint’ of human impact on recent climate changes. Credit: NASA

We have left a clear climate change ‘fingerprint’ in the atmosphere, through CO2 emissions that have made air near the Earth’s surface warmer and caused cooling higher up. That’s according to Ben Santer from Lawrence Livermore National Laboratory (LLNL) in California, who started studying this fingerprint in the mid-1990s, and his expert team. They have strengthened the case by comparing satellite-recorded temperature data against the latest climate models including natural variations within Earth’s climate system, and from the sun and volcanic eruptions. Ben hopes that in the process their results will finally answer ill-tempered criticism his earlier work attracted, and lingering doubts over what causes global warming.

“There are folks out there even today that posit that the entire observed surface warming since 1950 is due to a slight uptick in the Sun’s energy output,” Ben told me. “That’s a testable hypothesis.  In this paper we look at whether changes in the sun plausibly explain the observed changes that we’ve monitored from space since 1979. The very clear answer is that they cannot. Natural influences alone, the sun, volcanoes, internal variability, either individually or in combination, cannot explain this very distinctive pattern of warming.”

That pattern emerged when scientists in the 1960s did some of the first computer modelling experiments looking at what would happen on an Earth with higher CO2 levels in the air. “They got back this very curious warming in the lower atmosphere and cooling of the upper levels of the atmosphere,” Ben explained. The effect happens because most of the gas molecules in the atmosphere, including CO2, sit relatively near to Earth’s surface. CO2’s greenhouse effect lets heat energy from the Sun reach the Earth, but interrupts some of it getting back to the upper atmosphere and outer space. Adding more CO2 by burning fossil fuels therefore warms the lower atmosphere, or troposphere, and cools the stratosphere, 6-30 miles above the Earth’s surface.  Read the rest of this entry »

Google search basis undermines sunspot-winter coldness link

Franck Sirocko's 2012 study incorrectly dated this 1929 postcard identifying a year that the Rhine froze as being from 1963, which is one of many problems Geert Jan van Oldenborgh and his colleagues found with it. Image from van Oldenborgh et al, used under Creative Commons license, see citation below.

Franck Sirocko’s 2012 study incorrectly dated this 1929 postcard identifying a year that the Rhine froze as being from 1963, which is one of many problems Geert Jan van Oldenborgh and his colleagues found with it. Image from van Oldenborgh et al, used under Creative Commons license, see citation below.

European researchers have strongly criticised a recent study linking cold winters in the continent to cycles affecting the sun for relying on a shallow internet search. In August 2012, Franck Sirocko at University of Mainz, Germany, and his teammates linked cold years to sunspot activity lows using historical reports of when the river Rhine froze. But their results disagree with previous research, and previously unpublished findings from Geert Jan van Oldenborgh from KNMI, the Royal Netherlands Meteorological Institute, in De Bilt. And when Geert Jan looked into why this was, he found problems common in research on this topic over 50 years ago, updated for the internet age.

“These problems are fundamental – all the results that they claimed are spurious,” Geert Jan told me. “It is simply an incorrect paper. Usually incorrect results are just ignored, they do not get cited much and are quickly forgotten. However, this time we took the unusual step to write a comment on the paper. This decision was based on the low quality and the wide publicity it was given.”

That publicity came largely because the American Geophysical Union, which published the 2012 paper, put out a press release about it that the media reported widely. It tells how Franck’s team used historical documents to find that the Rhine froze in multiple places fourteen different times between 1780 and 1963. 10 of the 14 freeze years occurred close to the point in an 11 year cycle when there are fewest sunspots. “We provide, for the first time, statistically robust evidence that the succession of cold winters during the last 230 years in Central Europe has a common cause,” Franck said in the press release.

Sunspot cycles had been linked to weather throughout the 19th and 20th centuries, until Barrie Pittock started going over the evidence in the 1970s. Barrie, who led the Climate Impact Group at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia until his retirement in 1999, found no link beyond day-to-day weather effects. He also found many studies had used bad or incomplete data to say otherwise. Read the rest of this entry »

Projected warming set to exceed civilisation’s experience

Oregon State University's Shaun Marcott has built a climate record reaching back 11,300 years, showing that today's temperatures are warmer than at least 70% of that period. Credit: Shaun Marcott

Oregon State University’s Shaun Marcott has built a climate record reaching back 11,300 years, showing that today’s temperatures are warmer than at least 70% of that period. Credit: Shaun Marcott

The world is headed for average surface temperatures warmer than it has seen in at least 11,300 years. That’s one conclusion US researchers have reached after bringing together 73 studies of ancient climate from across the world into a single global record. Their work supports previous records for the past 2,000 years built mainly from tree ring data, explained Shaun Marcott from Oregon State University, and gives a much broader view.

“We can put today’s global temperature into context against the entire Holocene period,” Shaun told me. “That’s when human civilisation was born, developed and progressed to today.” Modern temperatures are higher than in around three-quarters of that period, which reaches back to the end of the last ice age. And their comparison against forecasts for 2100 made in models used by the Intergovernmental Panel on Climate Change (IPCC) is even starker. “If those scenarios come to fruition, we’ll be well outside anything human civilisation has seen,” Shaun warned. “We won’t have even have been close.”

Knowing climate’s history helps understand its present, and so researchers have puzzled out temperatures on the Earth’s surface from proxy, or indirect, records for the last 2,000 years. In particular, bringing together measurements from tree rings, ice and coral has showed a sharp recent temperature rise often referred to as the ‘hockey stick’. Meanwhile, studies scattered across the world had reached back across the 11,300 years since the beginning of the Holocene. But they can be influenced by regional effects, and no one had pieced them into a global view that would overcome that. Read the rest of this entry »

Evidence rethink puts CO2 and ancient warming back in sync

A thin layer of ice from an area of the Antarctic where ancient ice records are collected, in polarized light that reveals ice crystals. Rethinking how ice crystal formation affects ancient data collection is helping to solve an outstanding climate puzzle. © Frédéric Parrenin

A thin layer of ice from an area of the Antarctic where ancient ice records are collected, in polarized light that reveals ice crystals. Rethinking how ice crystal formation affects ancient data collection is helping to solve an outstanding climate puzzle. © Frédéric Parrenin

A different way to dig up links between past levels of CO2 in the air and temperatures could solve a troubling question over the historical climate. Previously, data collected from long cylinders drilled from Antarctica’s ice sheet seemed to show temperatures rising hundreds of years before CO2 levels did. If ancient warming came before a CO2 rise, then the greenhouse gas seemingly couldn’t have caused the warming. Climate skeptics have used this to argue  that the CO2 we produce today isn’t causing global warming.

Now, Frédéric Parrenin at the French National Centre for Scientific Research in Grenoble and his teammates have used a different method on these cylindrical ice cores. They say that their approach shows CO2 and temperature rises happened together during the last ‘deglaciation’, when ice sheets retreated during an abrupt warming period 20,000-10,000 years ago. “This makes it possible that CO2 was actually a cause of warming corresponding to the last deglaciation,” Frédéric told me.

Scientists have been using Antarctic ice cores, and bubbles of air from the time the ice formed trapped inside, to study climate history for over 30 years. The time capsule-like bubbles show what chemicals were in the air. Meanwhile, the amounts of different forms, known as isotopes, of elements like hydrogen, carbon and oxygen in the ice reveals the temperature it formed at. And finally, scientists figure out how old the ice and bubbles are from how deep they are in the core – and that’s where Frédéric found problems. Read the rest of this entry »