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.
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 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."
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 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.
Read also:
Part 2 - Is half a degree of warming really such a big deal?
Fact Check: Is Half a Degree of Warming Really Such a Big Deal? - Part II
Minimal temperature increases like 1.5 or 2 degrees Celsius might sound insignificant, but when it comes to our planet they can have a massive impact. DW looks at the facts. When a human being's temperature rises from a healthy 36.6 to 38.6 degrees Celsius (97.8 to 101.48 degrees Fahrenheit), it has consequences. Just a seemingly minor increase leaves the body feeling unwell and unable to function normally. It's a similar story for the planet. Since the late 19th century, when burning fossil fuels was becoming more widespread, the Earth has warmed by an average of more than 1 degree Celsius. Some places, however, have warmed beyond that level.
One of them is the Arctic. According to the Arctic Monitoring and Assessment Programme (AMAP), a working group of the intergovernmental Arctic Council, the average annual temperature in the region jumped by 3 degrees Celsius between 1971 to 2019. And that spells big problems for the region's ecosystem.
Small Temperature Rise = Big Species Losses
In a 2021 study published in The Cryosphere scientific journal, British researchers revealed findings indicating the loss of 28 trillion tons of ice between 1994 and 2017. They said the volume lost would be enough to cover the whole of the United Kingdom in an ice sheet 100 meters (328 feet) thick. Using satellite data to examine glaciers and the poles, scientists from the UK's University of Edinburgh, University College London and University of Leeds concluded that some 800 billion metric tons of ice was lost annually in the 1990s. By 2017, however, that number had risen to 1.2 trillion tons each year within that time span. Steven Amstrup, chief scientist at United States-based conservation nongovernmental organization Polar Bears International, has been researching wildlife in the Arctic since the 1980s and has seen the changes firsthand.
"Back then, I remember looking at the sea ice that was right offshore in northern Alaska in the middle of summer — it was never very far from the coast," he told DW. "Now during the same period, the sea ice is hundreds of miles offshore. If you'd told me at the beginning of my career that I was going to see these kinds of changes, I would have said you're crazy," Amstrup said.
In findings published in the Nature Climate Journal last year, he and his colleagues predicted that if the temperature keeps rising, most polar bears, which prey on seals resting on the ice, could disappear by the end of the century. "Sea ice is a polar bear's dinner plate," Amstrup said. "There's almost nothing as nutritious as seals for them to eat on land." The UN's Intergovernmental Panel on Climate Change (IPCC) predicts that if the planet warms 1.5 degrees Celsius above pre-industrial levels, the iconic polar species will still have sea ice during most Arctic summers. Under a 2 degrees scenario, however, they would face ice-free summers every 10 years.
Polar bears are not the only victims of rising temperature. Some 19% of species on the International Union for Conservation of Nature (IUCN) Red list face an increased likelihood of extinction due to climate change. At least one species has already been wiped out. In 2019, the Bramble Cay melomys, a tiny rodent that lived on a small sandy island at the tip of the Great Barrier Reef, became the first mammal to be officially recognized by the Australian government as extinct due to human-induced climate change. Rising sea levels and surging seas are believed to have wiped out its food and habitat.
Changes to our oceans are also impacting coral reefs, which serve as nurseries and pantries for up to a quarter of marine species. Warmer waters cause the corals to expel the sustaining marine algae from their tissues — known as bleaching. Prolonged bleaching kills the corals. A recent study found that Australia's Great Barrier Reef has lost half its corals since 1995. The IPCC has warned that if the planet warms beyond 2 degrees Celsius, corals will be almost entirely wiped out.
A World Beyond 1.5 degrees Celsius
Small temperature increases will also change life for humans. People will be exposed to more extreme weather — heat waves, droughts, floods and tropical cyclones. Their frequency and severity will likely depend on how high temperatures rise. If the world warms 2 degrees Celsius by 2100, the IPCC warns that 37% of the global population could be exposed to severe heat waves at least once every five years. Fewer than half of that number would be affected in a 1.5 degree scenario.
According to a 2018 study carried out by The Joint Research Center (JRC) of the European Commission's science and knowledge service, as the world warms, two-thirds of the population will experience a progressive increase in drought conditions. The UN Refugee Agency says increasing intensity of extreme weather events and sea-level rise are already causing more than 20 million people around the world each year to be internally displaced, or be forced to move to other parts of their country. Displacement is an even bigger issue for small island nations in the Pacific, Indian Ocean and Caribbean.
"Countries like the Marshall Islands can carry adoption plans for a certain sea level rise," said Helene Jacot Des Combes, a climate scientist at the University of the South Pacific, also an IPCC author and adaptation adviser to the Marshall Islands government. "But if it continues to go up, then these islands won't be inhabitable anymore." The Pacific island state of Fiji is also facing new realities. After being hit by 12 cyclones and other extreme weather events since 2016, the government launched a relocation program. More than 40 coastal communities need to be moved inland, and six of them have already been relocated.
Given the far-reaching consequences of just a slight temperature increase, the Paris climate accord ultimately aims to cap the global rise at 1.5 degrees Celsius in this century. But modeling suggests that based on current performance, the world is on track to reach that level of warming within the next 15 years. And without radical action today, temperature increases won't stop there. According to The Climate Action Tracker (CAT), an independent group of organizations that analyze governments' climate action, even if all current promises and plans worldwide were met on time, temperatures would still rise to 2 to 2.2 degrees Celsius by the end of this century. And the CAT considers that an optimistic outlook.
Part III, Is China the Main Climate Change Culprit?, see (here).
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