How can we be so sure that the Earth is heating?
Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, and sea level has risen.
Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report
“Join the dots. It’s happening. It’s happening in your world, it’s happening in my world. And let’s be very clear about this – it is going to get much worse.”
Dr Sunita Narain, Director General of The Centre for Science and Environment
Independent temperature records from multiple official sources confirm that there is absolutely no doubt whatsoever that the Earth is heating.
Each of the last three decades has been successively hotter than the one before, 19 of the top 20 hottest years have occurred in the last 19 years, and the past four years have been the hottest on record. 2016 was the hottest year ever recorded, whilst in 2019, nearly 400 temperature records were broken across 29 countries, June 2019 was the hottest on record, and July 2019 was the hottest month ever recorded. As of July 2020, January 2020 was the warmest January ever recorded in Europe, we saw the hottest May ever and we already have an 85% chance that 2020 will turn out to be the hottest year on record.
Some people argue that global heating can’t be happening because the weather seems to be getting colder where they live. This may indeed be true, but what’s important to remember is that when we talk about global heating and climate change, we are talking about changes in long-term average trends in atmospheric conditions – normally measured over decades – whereas when we talk about the weather we are referring to short-term and local variability around that average.
What this means is, whilst we are clearly seeing an overall increase in average global temperatures, there can still be significant regional and yearly variations in the weather. In any one year there may be some parts of the world that are colder than usual, and there may be entire years that are colder than previous years. Indeed, those wishing to confuse the public that global heating isn’t occurring sometimes do so by pointing to misleading graphs that are based on false or misinterpreted data that refers to regional as opposed to global changes, or cherry-pick data that focus on short-term trends rather than looking at the bigger picture.
The key point is that long-term and global trends show unequivocally that our planet is not only heating up, but that its rate of heating is accelerating. In fact, it has even been proposed that it is climate change itself that is causing some regions to experience more extreme, colder winters due to the disruption to weather patterns.
It’s also worth noting that there is variation in how fast the climate is changing in different parts of the world. For example continental regions (where most people live) heat up much faster than the oceans. An extreme example of this is the Arctic, which is warming at more than twice the rate of the global average due to a phenomenon called Arctic Amplification. In contrast, there are areas of the globe that are warming a bit slower than the average.
Why should we care about a few degrees of heating?
“It is generally foolish to bet against the judgments of science, and in this case, where the planet is at stake, it is insane.”
Professor Steven Weinberg, Nobel-Prize winning Theoretical Physicist, 2018
Over the past 12,000 years the Earth’s climate has been remarkably stable. It was this stability that gave the predictability and security that was needed for agriculture and human civilisation to develop, without which we could not have flourished. However, the burning of fossil fuels, large-scale deforestation and intensive farming practices that have been taking place since the start of the Industrial Revolution are causing us to now rapidly leave that period of climate stability behind and head into the unknown. As a result, we risk disrupting our planet’s environmental conditions way beyond that which we can adapt to. Indeed, we are already beginning to see the consequences – see section on what’s already happening to our planet as a result of global heating.
The average surface temperature of our planet has now risen by around 1°C since pre-industrial times. One degree may not sound like much, but the Earth is so massive that it takes a colossal amount of energy to warm it by even a tiny amount. In fact, the current rate of global heating is the equivalent energy of five nuclear bombs going off every second. Adding this much energy to the Earth’s climate inevitably has huge knock-on effects, causing extreme weather events across the globe, melting ice caps and disrupting life on land and in our oceans, which puts stress on agriculture and endangers fresh water supplies. The rise in global average temperature between the last ice age (over 17,000 years ago) and today is 4.5°C – yet we could see a similar amount of heating by the end of this century, in a fraction of the time.
At just 1°C of global heating, climate change is already here and is having devastating impacts on people and ecosystems across the world. Every fraction of a degree of further warming will not only exacerbate these impacts but also increase the risk of triggering irreversible tipping points. We are on course for things to get an awful lot worse over the coming decades unless we act radically and act NOW. Yet our governments continue to procrastinate and delay meaningful action.
Hasn’t the Earth been hotter in the past?
“You’d have to go back to the last interglacial [warm period between ice ages] about 125,000 years ago to find temperatures significantly higher than temperatures of today.”
Dr Carrie Morrill, paleoclimatologist at the National Climatic Data Centre
The planet is now very probably hotter than at any point in at least the last 125,000 years, long before human civilisation began, when sea levels reached over 6 m higher than they are today.
But what’s even more concerning is how fast our temperatures are rising. Over the past 45 years, our planet’s temperature has been increasing a whopping 170 times faster than the baseline rate of cooling over the previous 7,000 years. Indeed, our current rate of warming is unprecedented over the last 10,000 years.
Global temperatures over the past 11,300 years compared to the average between 1961 and 1990
Over the rest of this century, future temperature rises are predicted to be taking place not just much faster than it did during our recovery from the last ice age but hundreds of times faster than any extended period of warming in the last 65 million years. That’s when the dinosaurs went extinct. Crucially, when temperatures rise this fast, it is impossible for many living creatures and plants to have time to adapt to such changes. Not to mention the fact that many places on Earth that creatures would have previously used to take refuge from increasing temperatures have now been degraded, fragmented or colonised by human activities.
To make matters worse, geological records show that there have been some incidences of abrupt climate change in the past, when just a small change in one element of the climate system led to rapid changes in the whole system that have taken place over the course of centuries or even decades. This suggests the terrifying possibility that if we push global temperatures over a certain threshold today (which could happen at any time), we could trigger abrupt, unpredictable and potentially irreversible changes over the course of decades or even years – leading to large-scale impacts and massive disruption.
The bottom line is that the changes in temperatures that we’ve been seeing on our planet in recent years are truly unprecedented. At a time when sunspot activity is decreasing and when, according to the Earth’s natural orbital cycles, we ought to be cooling down and heading slowly towards our next ice age (see section on the effects of natural fluctuations in carbon dioxide), instead, greenhouse gases (such as carbon dioxide) from human activities are causing global temperatures to rise at a terrifying rate.
As our carbon dioxide levels reach concentrations not observed for millions of years (see section on what greenhouse gas emissions are like today), we will inevitably soon surpass any past climates experienced by humans.
What exactly are greenhouse gases and what is the greenhouse effect?
“Everything that is expected to result from global climate change driven by greenhouse gases is not only happening, but it’s happening faster than anybody expected.”
Professor John Holdren, Science and Technology advisor to President Barack Obama, Director of the Woods Hole Research Center
The temperature at the Earth’s surface is controlled primarily by the levels of certain gases in the atmosphere, such as water vapour, carbon dioxide and methane. Although these gases only make up a tiny fraction of our atmosphere (current carbon dioxide levels are around 410 parts per million (ppm), which is just 0.041%), just like how a drop of ink can affect the colour of a huge volume of water, tiny amounts of these gases can have enormous impacts on our atmosphere. Due to their special structure, these molecules can absorb heat emitted from the Earth’s surface as a result of it having been warmed by the Sun, preventing some of that heat from escaping back out into Space. In this way, it can be said that the gases provide an insulating ‘blanket’ around the Earth, which traps heat in our atmosphere keeping us warm.
This heating effect has (somewhat wrongly) been compared to how the glass roof of a greenhouse traps heat energy from the Sun, keeping the inside of the greenhouse warmer than its surroundings. Hence this phenomenon has become known as the greenhouse effect and the gases known as greenhouse gases. We have known about the greenhouse effect for well over 150 years, and the science behind it is well established.
The greenhouse effect, in its natural form, is essential for life here on Earth; without greenhouse gases in our atmosphere, the average temperature at the Earth’s surface would be around -18°C, dropping to temperatures at night that would be far too cold for us to survive. However, by burning fossil fuels (as well as by farming cattle, growing rice, burning trees, intensively ploughing soil and using chemical fertilisers) humans have been adding huge quantities of additional greenhouse gases into the atmosphere, ‘supercharging’ the greenhouse effect beyond anything that humans have ever experienced.
How the Greenhouse Effect works
Carbon dioxide is by far the most important greenhouse gas – responsible for approximately 75% of the Earth’s human caused heating – because it is emitted in much larger quantities than any of the other ‘long-lived’ greenhouse gases. It also has an extremely long lifetime: any excess can stay in the atmosphere for hundreds or even thousands of years and therefore it accumulates in our atmosphere, increasing its concentration. Concentrations of carbon dioxide have now increased by over 40%, from around 280 ppm in preindustrial times to over 414 ppm in July 2020 and rising.
Other greenhouse gases are important too though. For example, methane is a very potent greenhouse gas, being 86 times better than carbon dioxide at trapping heat over a twenty-year time frame. However, on average, it breaks down in the atmosphere in around 9 years, meaning that it can’t accumulate in the atmosphere as much as carbon dioxide over long time periods. Yet even over a 100-year time frame, methane is still approximately 34 times more potent than carbon dioxide. Plus, when methane breaks down it actually turns into carbon dioxide. Methane concentrations in the atmosphere have more than doubled since pre-industrial times, recently exceeding 1.8 ppm.
Per kilogram, nitrous oxide is an even more powerful greenhouse gas than methane, with nearly 300 times the global heating potential of carbon dioxide and a lifetime somewhere between that of carbon dioxide and methane – roughly 120 years. Levels of nitrous oxide have increased by around 22% since the preindustrial period.
Water vapour is also a greenhouse gas and occurs in our atmosphere in higher concentrations than carbon dioxide. However, the amount of water vapour in the atmosphere is controlled by the temperature of the air (increasing by around 7% with every degree celsius of increase in air temperature), rather than by human-caused emissions. For that reason, the amount of water vapour in the air is seen as an ‘amplifying feedback’ that increases global heating, rather than the initial cause of such heating.
Can we be certain that humans are causing global heating?
“Evidence for man-made warming of our climate system is unequivocal.”
Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report
There is now absolutely no doubt that the recent increase in global temperatures is almost entirely due to human factors. A vast body of peer-reviewed scientific evidence confirms that natural cycles, volcanic activity, galactic cosmic rays and changes in solar activity from sunspots have had a negligible effect on our current temperature rise. Instead, rises in global temperature have followed the trajectory of increased greenhouse gas levels.
Indeed, scientists have been able to find an unmistakable “human fingerprint” on climate change. Firstly, the Earth’s atmosphere is reacting exactly as we would expect it to if it were being exposed to an increase in greenhouse gases (as opposed to, for example, if it were experiencing increased solar activity). Secondly, detailed analysis of the carbon atoms in the carbon dioxide accumulating in the atmosphere shows that they must have been released by the burning of fossil fuels. Thirdly, measurements taken by satellites show that increases in greenhouse gases are trapping infra-red radiation from the Earth and causing the atmosphere to heat up.
Such evidence confirms that the direct impact of humans on our global climate is no longer a matter for debate. Indeed, an analysis of 12,000 academic papers on the subject of climate change published from 1991-2011 found that 97% of the papers that expressed a position agreed that humans are responsible for the exceptional levels of global heating that we are seeing today. To be clear, this doesn’t mean that the other 3% disagreed; it just means that their work didn’t meet the threshold to show that they agreed. The consensus has only gotten stronger: more recently, analysis of 11,602 peer-reviewed articles on “climate change” and “global warming” published in 2019 found 100% agreement that it is human activity that is responsible for this.
Whilst climate change deniers sometimes post articles or comments from scientists that seem to refute the overwhelming evidence of human-caused global heating, these articles often rely on inaccurate claims about climate science. The scientists themselves tend not to have expertise in a climate-related field, or they have links to the fossil fuels industry – or both.
The scientific community as a whole are overwhelmingly concerned about what is going on on our planet. In the words of Professor Lonnie Thompson, director of the Byrd Polar Research Centre: “Virtually all of us are now convinced that global warming poses a clear and present danger to civilization.”
Haven’t natural fluctuations in carbon dioxide affected the Earth’s temperature?
“At present rates of human emissions, there will be more CO2 in Earth’s atmosphere by 2025 than at any time in at least the last 3.3 million years.”
Dr Elwyn De La Vega, University of Southampton
In the absence of human activities, the levels of carbon dioxide in our atmosphere tend to remain fairly stable over time. Carbon dioxide is constantly being absorbed into the oceans, and locked into carbon compounds in trees and plants by a process called photosynthesis. Other living creatures feed off these trees and plants, taking the carbon compounds into their own bodies. These carbon compounds are then turned back into carbon dioxide through a process called respiration, and the carbon dioxide is released back into the atmosphere. Carbon dioxide is also released when dead matter decays and rots, or is burnt. This process of the constant removal and replacement of carbon dioxide in our atmosphere is known as the carbon cycle.
Whilst analysis of air bubbles trapped in Antarctic ice sheets reveals that over the past 800,000 years there also have been periods in the Earth’s history where the carbon dioxide levels have naturally risen and fallen, this has taken place extremely gradually. These changes are brought about by natural variations in the way in which the Earth travels around the Sun (known as Milankovitch cycles), which lead to changes in the absorption of sunlight on Earth. For example, a small increase in the amount of sunlight being absorbed causes the oceans to warm slightly, which results in some of the carbon dioxide dissolved within them being released back into the atmosphere. The increase in atmospheric carbon dioxide leads to more ocean warming, which causes more carbon dioxide to be released, which causes more warming… and so on and so forth. The changes in temperature caused by this feedback loop, and others like it, drive the transitions into ice ages (glacial periods) and back out of them (interglacial periods). The last time this happened was around 17,000 years ago, when we began to transition out of our most recent ice age.
The key point here is that these natural changes in carbon dioxide levels, triggered by cycles in the Earth’s orbit and modulated by feedback cycles within the Earth system, take place over tens of thousands of years. In contrast, the extremely rapid increases in carbon dioxide levels that we have been seeing over the past 60 years, due to human actions, have been taking place about 100 times faster than any of these previous natural increases.
Carbon dioxide levels today are higher than they have been in more than 3 million years. Back then, global average temperatures were 2-3°C higher than in pre-industrial times and sea levels were a whopping 16 m higher. Studying these past changes has revealed to climate scientists just how sensitive the Earth system is to changes in greenhouse gas concentrations. This is one of the key reasons why scientists are so alarmed about the current rapid rise in their concentrations.
What’s happened in the past few thousand years?
Around 17,000 years ago the Earth began to come out of its most recent ice age. As carbon dioxide levels naturally rose, the planet warmed. Then, about 12,000 years ago, global temperatures reached a plateau and we entered a period of relative climatic stability known as the Holocene. It is this stability that allowed humans to settle and farm. Then, around 5,000 years ago, greenhouse gas levels started to naturally fall again and temperatures began a slow decline, which would eventually have sent us towards our next ice age.
However, a few thousand years ago, human actions began to disrupt the Earth’s natural cycles. We started cutting down trees to clear land for farming or to burn them to keep warm – reducing the amount of carbon dioxide that could be removed from the atmosphere by photosynthesis. At the same time, we planted paddy fields to grow rice, releasing methane into the atmosphere from microbes growing in waterlogged soils. For a long period of time the levels of greenhouse gases in our atmosphere rose slowly and steadily. It has been proposed that this increasing greenhouse warming effect counteracted what should have been a period of natural cooling due to the Earth’s orbital cycles, leaving our global temperatures to remain pretty constant.
That is, until the last couple of centuries. With the start of the Industrial Revolution came mass burning of fossil fuels, large-scale deforestation and intensive farming. As a result, the levels of greenhouse gases began to shoot up astronomically. And with them, our temperatures.
What’s happened in the past 150 years?
“In just 100 years, fossil fuel use has more than undone 5000 years of natural cooling. It’s hotter now than any time in the history of human civilisation. We are catapulting ourselves out of the Holocene into uncharted territory. Current life on Earth is not adapted to this.”
Stefan Rahmstorf, Professor of Physics of the Oceans at Potsdam University
Since the Industrial Revolution, humans have been pumping enormous additional quantities of carbon dioxide into the atmosphere due to the burning of fossil fuels: coal, oil and natural gas. Fossil fuels are naturally occurring substances that were formed millions of years ago from the remains of dead plants and sea-creatures. When these fuels are burnt, the carbon compounds that have remained trapped underground for millions of years are converted into carbon dioxide and released, adding extra carbon dioxide into the atmosphere that would not naturally be there.
In addition, deforestation on massive scales to clear land for agriculture and livestock has released huge amounts of carbon dioxide back into the atmosphere, and has also meant that there are increasingly fewer trees to absorb excess carbon dioxide from the air (see section on deforestation).
At the same time as humans have been burning fossil fuels and clearing forests, they have also been churning up our planet’s soils through intensive farming practices. Healthy soils hold around 70% of the planet’s land-based organic carbon. However, when soil is repeatedly ploughed or compacted by heavy machinery or livestock, its ability to store carbon is compromised and vast quantities are released back into the atmosphere in the form of carbon dioxide (see section on soil degradation). The graph below shows just how strikingly carbon dioxide levels have changed over the past 1,000 years.
Carbon dioxide levels in the atmosphere over the past 1,000 years
It’s not just carbon dioxide levels that are increasing due to human actions. Methane levels have more than doubled in the last 150 years. Leaks from the oil and gas industry contribute heavily to the amount of methane in our atmosphere, but the waste sector and agriculture are also major sources. There has also been a huge increase in the number of rice-producing paddy fields and in the breeding of cattle for the meat and dairy industries. Microbes in the waterlogged soils of flooded rice fields release large quantities of methane, as do cows and sheep when they burp and fart due to the presence of similar kinds of microbes in their stomachs that help them to digest grass. In addition, fertilisers and animal waste produce large quantities of nitrous oxide, increasing its levels by around a third in the past 150 years. (See sections on intensive agriculture and livestock farming).
As our greenhouse gas emissions have risen, so too have global temperatures.
What are greenhouse gas emissions like today?
“More than half of all industrial emissions of carbon dioxide since the dawn of the Industrial Revolution will have been released since 1988”
Dr. Peter C. Frumhoff, Director of science and policy at the Union of Concerned Scientists
Due to a deadly combination of burning fossil fuels and changes in land use, we are currently pumping out a whopping great 110 million tonnes of carbon dioxide into the atmosphere every 24 hours. That’s over 40 billion tonnes a year! Over the past 60 years, the annual rate of increase in atmospheric carbon dioxide has been about 100 times faster than any previous natural increase in at least the last 800,000 years, such as those that occurred at the end of the last ice age around 12,000 years ago – see graph in section on how natural fluctuations in carbon dioxide have affected the Earth’s temperature.
Carbon dioxide concentrations are now over 414 parts per million and rising, an increase of over 45% on pre-industrial levels. That’s the highest level seen in at least the last 3 million years.
The scary thing is that because carbon dioxide stays in the atmosphere for such a long time, even if we completely stopped emitting it today we would not reverse the warming that it has caused. It’s like filling a bathtub with the plug in it: the water level will keep going up as long as the tap is on, but once you turn the tap off it won’t go down unless you remove water from the tub. Likewise the level of carbon dioxide and the heating it causes depends on the total amount of carbon dioxide emitted over time, and temperatures will not go back down even once we stop emitting carbon dioxide. If we want to reduce global temperatures, we will have to actively remove carbon dioxide from the atmosphere, something which might be impossible to do at the scale required.
This means that the only way we can definitely avoid catastrophic and irreversible damage to our planet is if we reduce carbon dioxide emissions to zero before we reach the level of heating that would cause such changes. This leads to the concept of a “carbon budget” – the maximum amount of carbon dioxide we can emit over the whole century if we want to stay below a certain temperature.
The picture is slightly better for the shorter-lived greenhouse gases, such as methane. Just like how in an unplugged bathtub the water level will drop as soon as the tap is turned off, if we stopped emitting methane today, the amount that it heats the atmosphere would drop almost immediately. This makes reducing methane emissions a very effective way to slow the rate of global heating.
However, there is much more carbon dioxide in the air than other greenhouse gases and it stays in the atmosphere for longer, so reducing carbon dioxide emissions is still the most important factor in determining how much the Earth heats up overall. So if we want to keep the global temperature rise to below 1.5°C, it is essential that we rapidly reduce global emissions of both short- and long-lived greenhouse gases.
Yet despite all the policies and pledges from the government, global greenhouse gas emissions continue to shoot up at an alarming rate. Whilst there has been a recent drop in emissions due to the COVID-19 pandemic, the short term effects on climate will be minimal, and unless concerted effort is taken to stop fossil fuel development in the recovery period it will only be a temporary dip in a long-term upward trend. If this is a hole we need to get out of, we’re still digging.
Indeed, Fatih Birol, executive director of the International Energy Agency and one of the world’s foremost energy experts has warned that we have to act quickly if we want to change the course of the climate crisis and prevent a post-lockdown rebound in greenhouse gas emissions that would overwhelm efforts to stave off climate catastrophe. Birol said: “This year is the last time we have, if we are not to see a carbon rebound.”
We have been warned over and over!
“Future generations are unlikely to condone our lack of prudent concern for the integrity of the natural world that supports all life”
Rachel Carson, Silent Spring, 1962
The risks of climate change and ecosystem collapse as a result of human action have been known about for many decades. Yet global greenhouse gas emissions have continued to rise steeply.
In 1949, Professor Aldo Leopold, ecologist at the University of Wisconsin, made clear the damage we were doing to our land: “One of the penalties of an ecological education is that one lives alone in a world of wounds. Much of the damage inflicted on land is quite invisible to laymen. An ecologist must either harden his shell and make believe that the consequences of science are none of his business, or he must be the doctor who sees the marks of death in a community that believes itself well and does not want to be told otherwise.”
Global warming and its associated risks have been known about for over 40 years. Back in the 1970s, American multinational oil and gas corporation Exxon were aware of the likelihood and risks of human caused climate change.
In 1988, climate scientist James Hansen testified before the US congress, provoking the New York Times headline: “Global Warming Has Begun, Expert Tells Senate”. That same year, the Intergovernmental Panel on Climate Change (IPCC) – a massive body of thousands of scientists who volunteer their time to review the science relating to climate change – was founded as part of the United Nations to inform international negotiations. Since then the IPCC has published five massive reports, and many more interim reports, detailing the science of climate change in ever-starker detail.
Yet the warnings were ignored or down-played.
In 1992, the Union of Concerned Scientists, including the world’s leading scientists and the majority of living science Nobel laureates, published the “World Scientists’ Warning to Humanity”, calling on humankind to curtail environmental destruction and warning that “a great change in our stewardship of the Earth and the life on it is required, if vast human misery is to be avoided.” The authors feared that humanity was pushing Earth’s ecosystems beyond their capacities to support the web of life, and described how we are fast approaching many of the limits of what the biosphere can tolerate without substantial and irreversible harm. They implored that we cut greenhouse gas emissions and phase out fossil fuels, reduce deforestation, and reverse the trend of collapsing biodiversity. Indeed, a few years later, in 1995, the UN Environment Programme also warned that the Earth’s biological resources were under serious threat.
The warnings were still not heeded. On the contrary, the rate of greenhouse gas accumulation in the atmosphere between 1987-2019 (since when the dangers were first known) was four and a half times faster than it was between 1955-1986. In fact, more than half of our total fossil fuel emissions have been emitted in the last three decades. The graph below shows just how rapidly and how consistently our emissions have been rising, even since various climate warnings have been issued and climate treaties negotiated.
In 2017 there followed a second, even more urgently worded letter from the Union of Concerned Scientists: “World Scientists’ Warning to Humanity: A Second Notice”, which was signed by more than 15,000 scientists. A few years later, the urgency was made crystal clear by more than 13,000 scientists from 156 countries who signed the “World Scientists’ Warning of a Climate Emergency”. In it they declared that “Scientists have a moral obligation to clearly warn humanity of any great existential threat … Based on this obligation and the data presented below, we herein proclaim … a clear and unequivocal declaration that a climate emergency exists on planet Earth.”
Now, after almost 40 years of warnings and 30 years of international climate negotiations, we have precious little to show for it. On the contrary, carbon emissions from burning fossil fuels have increased by over 50% since 1990. If we are to prevent disaster the time for action is now.
As Professor Sherwood Rowland, Nobel Prize Winner in Chemistry for discovering how we were destroying the ozone layer said: “What’s the use of having developed a science well enough to make predictions if, in the end, all we’re willing to do is stand around and wait for them to come true?”