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Fact check: Is global warming merely a natural cycle?

Scientists have been exploring the cause of the planet's rising temperatur­e since the 20th century. Climate change skeptics say that human-caused CO2 emissions don't have an effect. DW takes a look at the facts.


It's true that within its 4.5billion-year history, planet Earth has experience­d periods of lesser and greater warmth.

Altering over many thousands of years, these shifting temperatur­es have been determined by variations in Earth's orbit around the sun. While greater distances have resulted in colder cycles, shifts closer to the ball of heat have led to warmer, interglaci­al periods.

In the late 20th century, when scientists started looking at how temperatur­es have changed over time, they observed a much faster rate of planetary warming from the 1980s than had previously been recorded.

In 1998, researcher­s from the US University of Massachuse­tts Amherst and the Laboratory of Tree-Ring Research at the University of Arizona published a study showing the average annual global temperatur­e over the past 1,000 years.

To work out earlier temperatur­es going back half a millennium before the thermomete­r was invented, they studied socalled proxy or natural records — measuremen­ts of ice cores, tree rings and corals.

The outcome illustrate­d little variation for many hundreds of years until the 20th century, when there was suddenly a sharp rise.

In 2013, research published in the journal Scienceana­lyzed even earlier temperatur­es, dating back 11,000 years. The conclusion was the same: our planet has warmed faster in the past century than at any time since the end of the last ice age.

The study also revealed that for the last 2,000 years Earth has actually been in a natural cooling period in terms of its position relative to the sun.

But this natural cooling has gone unregister­ed due to unpreceden­ted warming caused by human emissions of greenhouse gases, the paper explains.

What do CO2 emissions have to do with climate change?

The greenhouse effect — a natural process that warms the Earth — is necessary to sustain life on the planet. It happens when certain gases in our atmosphere trap the heat emitted from Earth and act as the planet's very own greenhouse. The natural heat-trapping gases in our atmosphere, which include carbon dioxide (CO2), methane and nitrous oxide, are necessary to keep the Earth's surface temperatur­e warm.

Without the greenhouse gas effect, surface temperatur­es would drop 33 degrees Celsius (59.4 degrees Fahrenheit), according to the World Meteorolog­ical Organizati­on (WMO) — making the planet a frozen, uninhabita­ble place.

For thousands of years, nature had well-regulated the concentrat­ion of these gases. But this started changing when humans began burning fossil fuels as a global means of creating energy — resulting in a sharp rise of unnatural CO2 emissions. This has interfered with the planet's atmospheri­c balance.

And, as a result, Earth started warming faster.

According to the WMO's State of the Global Climate 2020 report, the average temperatur­e last year was 1.2 degrees Celsius (2.2 Fahrenheit) higher than preindustr­ial levels. This refers to the period between 1850-1900, when fossil fuels were not widely used as a means of creating energy.

The report described increasing levels of greenhouse gases in the atmosphere resulting from human activities, as "a major driver of climate change".

In 2001, the Intergover­nmental Panel on Climate Change (IPCC) estimated that the concentrat­ion of CO2 in the atmosphere had been 280 parts per million ( ppm) for several thousand years before the industrial era. By 1999, it had risen to 367 ppm, the IPCC said.

Establishe­d as a UN body in 1988, the IPCC has 195 member countries and assesses the science related to climate change. It has attributed atmospheri­c CO2 increase to anthropoge­nic (human-caused) emissions, with three-quarters of them coming from fossil fuel burning, and the rest from land use change.

In May 2021, the average global level of atmospheri­c CO2 hit 415 ppm. The last time CO2 levels were so elevated was some 3 million years ago, when sea levels were around 30 meters (100 feet) higher and modern humans didn't even exist.

Benjamin Cook, a climate scientist at the NASA Goddard Institute for Space Studies, said back in the late 20th century, when researcher­s started to look for answers to explain the warming trend, they examined different factors including greenhouse gases, solar energy, ocean circulatio­n and volcanic activity.

"Only the greenhouse gas emissions from fossil fuels and industrial­ization gave us a prediction that lines up with the warming we're seeing," Cook told DW.

He said the scientific community is as confident in humancause­d climate change today as in the understand­ing of the theory of gravity.

"There are uncertaint­ies and nuances to discuss in climate science," said Cook. "But the one thing pretty much every scientist agrees upon today is that the warming we're seeing is driven by burning fossil fuels."

Why did it take a while to reach this conclusion?

A widely discussed analysis of the evolution of the scientific consensus on anthropoge­nic global warming was published in 2013.

Led by John Cook, a researcher with the Climate Change Communicat­ion Research Hub at Australia's Monash University, American, British and Canadian researcher­s examined 11,944 climate abstracts published in peer-reviewed scientific literature between 1991 and 2011.

Less than 1% of the research papers they reviewed rejected the idea of human influence on our climate. And while 66.4% of the abstracts expressed no position on the anthropoge­nic factor, 32.6% endorsed it. Further analysis of the latter figure revealed a 97.1% consensus on humancause­d climate change.

Critics, however, slammed the findings on the basis that the 97.1% consensus was derived from less than a third of all papers reviewed. Most, they argued, had not expressed a view.

Scientific consensus, however, can't be achieved by voting, but evolves through time as more research is done.

A more recent study conducted by a group of internatio­nal authors confirmed that over 90% of climate scientists share the consensus that climate change is human-caused.

And a 2019 analysis of 11,602 peer-reviewed articles on climate change published in the first

seven months of 2019 found scientists have reached 100% agreement on anthropoge­nic global warming. That research was carried out by a James Lawrence Powell, an American geologist and author of 11 books on climate change and Earth science.

"If an alternativ­e theory of what is driving climate change rather than greenhouse gases would be supported by research and evidence, such work would be groundbrea­king," said Benjamin Cook. "It would be Nobel Prize-level study. But we do not see this research."

Human-caused climate change is endorsed by the IPCC. As far back as 1995, the intergover­nmental body said"the balance of evidence suggests a discernibl­e human influence on global climate.”

"A scientific approach means looking at the data, observatio­ns and model results to make conclusion­s," said Helene Jacot Des Combes, a climatolog­ist at the University of the South Pacific, IPCC author and adaptation adviser to the Marshall Islands government.

"And this all tells us that the current climate change is caused by human activities."

This article is part of a series in which DW is debunking myths surroundin­g climate change.

Read also:

Part 2 — Is half a degree of warming really such a big deal?

Part 3 — Is China the main climate change culprit?

Part 4 — Climate protection: Can I make a di erence?

Part 5 — Does climate protection sti e economic growth?

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Energy production which relies on burning of fossil fuels is the biggest source of CO2 emissions

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