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 4.5-billion-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 so-called 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 Science analyzed 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.
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 pre-industrial 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 human-caused 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."
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 human-caused 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.