By Dr Catriona Nguyen-Robertson MRSV
Senior Editor, Science Victoria

Last year (2023) was the warmest year on record, with the global average temperature 1.35°C above the pre-industrial average (1850-1900).¹ How much of this warming can be attributed to human activity?

According to the latest Intergovernmental Panel on Climate Change (IPCC) Report: ‘Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with global surface temperature reaching 1.1°C above 1850–1900 in 2011–2020.’² The report does not shy away from the fact that ‘observed warming is human-caused’.

There are many factors that contribute to climate change – both in the past and in the present. Greenhouse gases in the atmosphere act like a blanket, trapping warmth. Without them, the temperature would be a cool -18°C, unable to sustain life as we know it.³ They were present before humans came along; however, many things in our modern world produce extra greenhouse gases, like the burning of fossil fuels.

Compared to other processes that produce greenhouse gases or warm our planet, how much are we to blame?

Warmth from the Earth’s core

The Earth formed around 4.5 billion years ago as rock, dust, and gas clumped together into a hot, giant ball. It was only during the first 50-100 million years that the Earth cooled down enough from its initial 1,500°C to form a solid mantle and outer crust, still with molten magma at its core.

Residual heat from the planet’s formation, as well as heat released from the radioactive decay of elements (e.g. uranium, thorium, and potassium) in the planet’s crust and mantle are sources of internal heat. They contribute a mere 0.03% of warmth to the atmosphere compared to incoming heat from the Sun.⁴ 

This heat from the Earth’s core dissipates into the atmosphere extremely slowly – too slowly to cause the warming we are seeing.

Basking in sunshine

An enormous amount of solar radiation is released from the Sun, reaching Earth as heat energy. Some heat is absorbed, especially by oceans, and most is reflected, radiating back out to space. Over the period of an 11 year-cycle, solar radiation levels fluctuate by up to 0.15%.⁵ 

Global surface temperature changes have fluctuated slightly with this cycle in the past, but since the late 20^th century, the global surface temperature has deviated on an upwards trajectory, unaffected by the 11-year cycle.⁵ 

The change is not large enough to cause any long-term changes to Earth’s climate.

An orbital dance of the Sun and Earth

Small variations in how Earth moves around the Sun influence its climate over great timespans. Three types of variations in Earth’s orbital movements, known as the Milankovitch cycles, affect how much solar radiation reaches the top of Earth’s atmosphere and where it reaches.⁶

Earth’s annual lap around the Sun isn’t perfectly circular. Over time, the pull of gravity from Jupiter and Saturn causes Earth’s orbit to shift from nearly circular to slightly elliptical, affecting the distance between the Sun and Earth.⁶ 

The angle of Earth’s tilt also varies. The greater the tilt, the more extreme our seasons are, as each hemisphere either receives more solar radiation during summer when it is tilted towards the Sun, or less when it points away in winter.⁶ 

Lastly, as Earth rotates, it wobbles slightly upon its axis – like an off-balance spinning top. This makes seasonal contrasts more extreme in one hemisphere (currently the southern hemisphere) and less extreme in the other.⁶

All three Milankovitch cycles span timeframes of tens to hundreds of thousands of years. While they are likely responsible for triggering the beginning and end of glaciation periods (Ice Ages), they cannot account for Earth’s current period of rapid warming.

Asteroid impacts

Most meteorites that reach Earth are small, and burn up in the atmosphere. But when a large asteroid smashes into the Earth, it ejects an enormous amount of dust, ash, and other material into the atmosphere. Asteroids with a diameter bigger than 1 km strike Earth roughly every several hundred thousand years.⁷ The greater the asteroid, the greater the time span in between.

The last known impact of an object of 10 km or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago – the asteroid that accelerated the demise of the dinosaurs. It slammed into rocks rich in carbonates, releasing immense quantities of carbon dioxide (CO₂), and triggered vast wildfires, releasing even more. Global temperatures rose by 5°C, and the Earth stayed that hot for 100,000 years.⁸ 

But we haven’t had a major asteroid impact like that for millions of years – not recently enough to cause the global warming we see now.

Volcanic gas and ash emissions

Gases and dust particles thrown into the atmosphere during large volcanic eruptions influence climate, creating both heating and cooling effects. Dust and ash particles spewed from volcanoes high into the atmosphere can shade incoming solar radiation, causing a temporary cooling effect. The catastrophic volcanic eruption of Mount Tambora in Indonesia in 1815 released 60 megatonnes of gases, ash, and other rock and aerosols into the atmosphere, plunging the entire world into a three-year winter (0.7°C) before temperatures began to rise again.⁹ 

The “super-eruption” of another Indonesian volcano, Toba, 74,000 years ago has been the largest known natural disaster in the past 2.5 million years.¹⁰ Toba ejected so much volcanic ash into the atmosphere that it cooled the world by 3-5°C.¹¹

Volcanoes can also have a warming effect when eruptions spew greenhouse gases, like CO₂ and sulphur dioxide, into the atmosphere. Over millions of years this may contribute global warming, but the annual flux of CO₂ emissions from volcanoes is under 260 million tonnes per year – much less than the 40 billion tonnes per year from human activities.¹² 

There have been no major greenhouse gas-producing volcanic eruptions in the last 250 years – volcanoes can’t be blamed for warming since the pre-industrial era.

Biological processes

We breathe in oxygen (O₂) and breathe out carbon dioxide (CO₂). Most life on the planet undergoes metabolism, consuming oxygen to survive. However, some organisms also go the opposite way: taking in CO₂ for photosynthesis.

Before photosynthesis evolved, oxygen levels in the atmosphere were very low and CO₂ levels were high. In Earth’s early days, the atmosphere was composed of gases from volcanoes: hydrogen sulphide, methane, and 10-200 times as much CO₂ as today’s atmosphere.¹³ Primitive photosynthesis did not produce oxygen, but as photosynthetic algae evolved around 2.5-1 billion years ago, oxygen started to build up in the atmosphere while carbon dioxide fell. Eventually, the amount of oxygen present in the atmosphere enabled animals to evolve. 

But changes in atmospheric gases levels through biological processes are slow, as they are long-term shifts in response to evolution – they have not contributed to current warming. That said, deforestation and land clearing have led to increased CO₂ by way of removing trees that would otherwise act as a carbon sink.

Humans

Lastly, we come to us: the anthropogenic (human-made) source of greenhouse gases. Three culprit gases have increased dramatically since the industrial revolution and contribute to global warming: carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). While “carbon emissions” are often discussed in the context of climate change and climate action, methane has over 28 times the heat trapping capacity of carbon dioxide, and nitrous oxide has 265 times the capacity.¹⁵ 

Anthropogenic carbon dioxide can be distinguished from natural sources based on isotopes – or chemical variants – of carbon: the lightest and most common version of carbon (¹²C), and heavier variant (¹³C). When we burn fossil fuels, we release a large amount of carbon dioxide containing the lighter (¹²C) carbon isotope, which tips the ratio of the two variants in the atmosphere. Since the beginning of the industrial era in 1750, and much more rapidly since 1950, this ratio has been increasingly favouring the lighter variant – correlating with the increased burning of fossil fuels.¹⁶

We therefore know that anthropogenic carbon dioxide has increased – and it is the major cause of the increasing atmospheric carbon dioxide.

If we are to blame, what can we do about it?

The United Nations ActNow campaign encourages people to save energy and change energy sources at home, eat plant-based foods, consider travel options, and reduce, reuse, repair and recycle.¹⁷

All of these suggestions are ‘good’, and as more individuals spend their money on these options, companies will respond to the changing consumer habits. If everyone in a town suddenly became vegetarian and only rode bicycles, you’d expect to see more greengrocers and bike stores pop up (while butchers and car dealers would have to adapt).

However, this thinking puts the onus solely on individuals. These individual changes can be good, but they’re nowhere near enough to meaningfully impact climate change. 

We need collective action at all levels – local, state, national, and international.

Policymakers may have established agreements and targets to cut emissions and limit warming (e.g., the Paris Agreement), but we need strong action to meet these targets. We need multiple solutions, and for those solutions to be enacted.

Australia currently hits hard in terms of climate change: our carbon emissions are higher than 90% of countries, and among the highest per capita in the world.¹⁸ The easiest, most efficient and cost-effective ways for Australia to reduce greenhouse gas emissions are:

• Transitioning away from fossil fuel generated electricity to renewable energy and storage technologies.
• Electrify our transport systems (and use renewable energy to power them).
• Reforestation and regenerative agriculture would counteract the roughly 13% and 9% that agriculture and deforestation currently contribute to emissions.
• Lastly, we actively need to transition away from both using and exporting fossil fuels.

Individuals can make individual changes, but we also need to pressure decision-makers to create change too. We can vote for policymakers who will enter into climate negotiations and adopt policies that protect the planet. We can join others to pressure governments to end fossil fuel subsidies, and to support key industries to transition to sustainable practices. We can ride a bike, and remember our reusable coffee cups, but we also need to write to the people we elect to act in our planet’s best interests.

Earth has experienced climate change before. These changes have been slow, caused by continental migration, Earth’s orbital behaviour around the Sun, volcanism, and evolution. Global temperatures have risen and fallen – often accompanied by mass extinction events. While we may give ourselves the benefit of the doubt, scientific evidence points to humans as the key drivers of our currently changing climate. It’s up to us to consider how we will address it.

This article was inspired by a presentation to the Royal Society of Victoria delivered by Professor Raymond Cas (Monash University) in September 2023. You can watch his presentation in full at youtu.be/e183_0Sr1c (or in brief at youtu.be/hjLx4P0dkCY).

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