It is true that in its 4,5 billion year long history, planet Earth has experienced periods of less and greater warming.
Temperature changes over thousands of years are determined by variations in the Earth's orbit as it orbits the Sun.
Although greater distances resulted in cooler cycles, shifts toward the hot ball led to warmer, interglacial ages.
At the end of the 20th century, when scientists began to study how temperatures have changed over time, they noticed that since the 1980s, planet Earth has been warming much faster than previously recorded.
In 1998, researchers from the University of Massachusetts and the Tree Ring Laboratory at the University of Arizona published a study showing the average annual global temperature over the past 1.000 years.
To calculate the temperatures that prevailed half a millennium before the thermometer was invented, they studied so-called proxies or natural records - measurements of ice cores, tree rings and corals.
The results showed little variation over hundreds of years until the 20th century, when warming suddenly spiked.
In 2013, in a study published in the journal "Nauka" (Science), temperatures dating back 11.000 years were analyzed.
The conclusion was the same: Planet Earth has warmed faster in the past century than at any time since the end of the last ice age.
The study also found that the Earth has been in a natural cooling period in terms of its position relative to the Sun for the past 2.000 years.
But this natural cooling has not been registered due to the unprecedented warming caused by the human factor, i.e. emissions of gases that cause the greenhouse effect, the paper explains.
What do CO2 emissions have to do with climate change?
According to the World Meteorological Organization (WMO) report on the state of the global climate 2020, the average temperature last year was 1,2 degrees Celsius higher than the temperature that prevailed in the pre-industrial era.
It refers to the period between 1850 and 1900 when fossil fuels were not widely used as a means of obtaining energy.
The report described increasing levels of greenhouse gases in the atmosphere as a result of human activities which are "the main driver of climate change".
In 2001, the Intergovernmental Panel on Climate Change (IPCC) estimated that several thousand years before the industrial era, the concentration of CO2 in the atmosphere was 280 parts per million (ppm).
By 1999, it had risen to 367 ppm, the IPCC said.
Founded as a body of the United Nations (UN) in 1988, the IPCC has 195 member countries and provides scientific assessments related to climate change.
The increase in atmospheric CO2 has been attributed to anthropogenic emissions, three-quarters of which comes from burning fossil fuels, and the rest from land-use change.
In May 2021, the average global level of CO2 in the atmosphere reached 415 ppm.
Benjamin Cook, a scientist and climate expert at NASA-Goddard Institute for Space Studies, says that in the late 20th century, when researchers began looking for answers to explain the warming trend, they looked at a variety of factors, including greenhouse gas emissions, solar energy, ocean circulation and volcanic activity.
"Only emissions from fossil fuels and industrialization have given a prediction that matches the warming we're seeing," Cook told DW.
He says that the scientific community today is certain that climate change is caused by humans, just as it is certain of understanding the theory of gravity.
"There are uncertainties and nuances that are debated in the field of climate science," Cook notes.
"But the one thing every scientist agrees on today is that the warming we're seeing is caused by the burning of fossil fuels."
Why did it take a while to come to this conclusion?
A widely debated analysis of the evolution of the scientific consensus on anthropogenic global warming was published in 2013.
American, British and Canadian researchers, led by John Cook of the Climate Change Research and Communication Network at Australia's Monash University, examined 11.944 climate abstracts published in the scientific literature between 1991 and 2011.
Less than 1% of the research papers they reviewed reject the idea of human influence on our climate.
66,4% of publications do not express any opinion on the anthropogenic factor, while 32,6% of works recognize the influence of the human factor on the climate.
Further analysis of the latter figure revealed a 97,1% consensus on man-made climate change.
Critics, however, believe that the 97,1% agreement was derived from less than a third of all papers reviewed.
The majority, they argued, had not expressed their opinion on the matter.
They also believe that scientific consensus can develop only with time and an increase in the number of conducted research.
A recent study conducted by a group of international authors confirmed that over 90% of climate scientists agree that climate change is man-made.
And a 2019 analysis of 11.602 peer-reviewed articles on climate change, published in the first seven months of that year, found that scientists were in XNUMX percent agreement on anthropogenic global warming.
That research was conducted by James Lawrence Powell, an American geologist and author of 11 books on climate change and geoscience.
"If an alternative theory of what drives climate change, rather than the greenhouse effect, were supported by research and evidence, such work would be revolutionary," said Benjamin Cook.
"It would be a study at the level of a Nobel Prize. But it is not in sight for now."
The IPCC supports the thesis that humans are behind climate change.
Back in 1995, this intergovernmental body said that "the balance of evidence suggests that there is a detectable human influence on the global climate."
"The scientific approach means summarizing data, observation, statistics and climate models in order to draw conclusions," says Helene Jacot de Combe, a climatologist at the University of the South Pacific and an author at the IPCC and concludes: "All this tells us that the current climate change is caused by human activities."
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