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Even with all humanity’s carbon emissions to date, there’s a lot less carbon dioxide in Earth’s atmosphere than Venus, and Earth is further away from the Sun. But if carbon emissions continue at the current rate, is there any risk of reaching a tipping point at which a runaway greenhouse effect takes over, making Earth uninhabitable for any form of life?
When sunlight enters the Earth’s atmosphere, some is reflected back to space by clouds, some is reflected by bright surfaces such as ice and snow and some is absorbed by the land surface and ocean.
To maintain a balance, the Earth emits energy back to space in the form of infrared, or longwave, radiation. Some longwave radiation is absorbed in the atmosphere by heat-trapping gases, such as carbon dioxide.
This is the well-known greenhouse effect.
As is already well established, concentrations of carbon dioxide have increased over the past 250 years, causing the average surface temperature to increase.
One consequence of increasing atmospheric carbon dioxide concentrations is that, as the atmosphere warms, it can contain more water vapour. Since water vapour is itself a greenhouse gas, this can create an amplifying effect.
In general, as surface temperature increases, the Earth emits more longwave radiation to space to maintain the energy balance. But there is a limit to how much longwave radiation can be emitted.
If the atmosphere becomes completely saturated with water vapour, the Earth’s surface and lower atmosphere warm up, but further increases in emission of longwave radiation are not possible.
The runaway greenhouse
This is termed a runaway greenhouse and would mean the Earth would become lethally hot and unable to cool itself by emitting heat to space.
Ultimately, this is the fate of the Earth. In billions of years from now the Sun will become brighter and grow into a Red Dwarf. As the Sun’s luminosity increases, the Earth will become hotter and its oceans will evaporate.
The hot and steamy atmosphere will ensure the Earth is just as uninhabitable to current life-forms as Venus is today.
But could we bring such a situation about on a shorter timeframe through continued carbon dioxide emissions? The good news is, probably not.
We’re safe, for now
Previous research has found that, due to differences in the properties of water vapour and carbon dioxide as greenhouse gases, adding carbon dioxide to the atmosphere is likely insufficient to trigger a runaway greenhouse.
In geological terms, this is a very large increase to take place over a short period of time. Yet human emissions of carbon dioxide are considered insufficient to trigger a runaway greenhouse, given the fossil fuel reserves available.
The Earth should be safe from a runaway greenhouse developing for at least another 1.5 billion years.
But then …
The caveat to all the above is that the models scientists use to study future climate are built based on past, known conditions. It is therefore difficult to predict how certain parts of the climate system might operate under extremely high greenhouse gas emissions scenarios.
For example, clouds can reflect sunlight back to space, or they can trap heat emitted by the Earth. In a warming world, scientists are still unclear on the role clouds will play.
While a runaway greenhouse would make Earth completely uninhabitable to life as we know it, the losses that may accrue from just a few degrees Celsius of global warming are serious and must not be discounted.
The silver lining is we (probably) don’t need to worry about becoming like our neighbour Venus any time soon.
You’d think it would be a simple piece of biological accounting – how many distinct species make up life on Earth?
But the answer may come as a bit of a shock.
We simply don’t know.
We know more accurately the number of books in the US Library of Congress than we know even the order or magnitude – millions and billions and so on – of species living on our planet, wrote the Australian-born ecologist Robert May.
That’s a massive degree of uncertainty. It’s like getting a bank statement that says you have between $5.30 and $1 million in your account.
So why don’t we know the answer to this fundamental question?
It’s hard to count life
Part of the problem is that we cannot simply count the number of life forms. Many live in inaccessible habitats (such as the deep sea), are too small to see, are hard to find, or live inside other living things.
So, instead of counting, scientists try to estimate the total number of species by looking for patterns in biodiversity.
In the early 1980s, the American entomologist Terry Erwin famously estimated the number of species on Earth by spraying pesticides into the canopy of tropical rainforest trees in Panama. At least 1,200 species of beetle fell to the ground, of which 163 lived only on a single tree species.
Assuming that each tree species had a similar number of beetles, and given that beetles make up about 40% of insects (the largest animal group), Erwin arrived at a controversial estimate of 30 million species on Earth.
Many scientists believe the 30 million number is far too high. Later estimates arrived at figures under 10 million.
In 2011, scientists used a technique based on patterns in the number of species at each level of biological classification to arrive at a much lower prediction of about 8.7 million species.
All creatures great and very, very small
But most estimates of global biodiversity overlook microorganisms such as bacteria because many of these organisms can only be identified to species level by sequencing their DNA.
As a result the true diversity of microorganisms may have been underestimated.
After compiling and analysing a database of DNA sequences from 5 million microbe species from 35,000 sites around the world, researchers concluded that there are a staggering 1 trillion species on Earth. That’s more species than the estimated number of stars in the Milky Way galaxy.
But, like previous estimates, this one relies on patterns in biodiversity, and not everyone agrees these should be applied to microorganisms.
It’s not just the microorganisms that have been overlooked in estimates of global biodiversity. We’ve also ignored the many life forms that live inside other life forms.
Most – and possibly all – insect species are the victim of at least one or more species of parasitic wasp. These lay their eggs in or on a host species (think of the movie Aliens, if the aliens had wings). Researchers suggest that the insect group containing wasps may be the largest group of animals on the planet.
What do we mean by species?
A more fundamental problem with counting species comes down to a somewhat philosophical issue: biologists do not agree on what the term “species” actually means.
The well-known biological species concept states that two organisms belong to the same species if they can interbreed and produce fertile offspring. But since this concept relies on mating, it cannot be used to define species of asexual organisms such as many microorganisms as well as some reptiles, birds and fish.
It also ignores the fact that many living things we consider separate species can and do interbreed. For example, dogs, coyotes and wolves readily interbreed, yet are usually considered to be separate species.
Other popular species definitions rely on how similar individuals are to one another (if it looks like a duck, it is a duck), their shared evolutionary history, or their shared ecological requirements.
Yet none of these definitions are entirely satisfactory, and none work for all life forms.
There are at least 50 different definitions of a species to choose from. Whether or not a scientist chooses to designate a newly found life form as a new species or not can come down to their philosophical stance about the nature of a species.
The cost of species loss
Our ignorance about the true biodiversity on our planet has real consequence. Each species is a potential treasure trove of solutions to problems including cures for disease, inspirations for new technologies, sources of new materials and providers of key ecosystem services.
Yet we are living in an age of mass extinction with reports of catastrophic insect declines, wide-scale depopulation of our oceans and the loss of more than 50% of wildlife within the span of a single human life.
Our current rate of biodiversity loss means we are almost certainly losing species faster than we are naming them. We are effectively burning a library without knowing the names or the contents of the books we are losing.
So while our estimate of the number of species on the planet remains frustratingly imprecise, the one thing we do know is that we have probably named and described only a tiny percentage of living things.
New species are turning up all the time, at a rate of roughly 18,000 species each year. For example, researchers in Los Angeles found 30 new species of scuttle fly living in urban parks, while researchers also in the US discovered more than 1,400 new species of bacteria living in the belly buttons of university students.
Even if we take the more conservative estimate of 8.7 million species of life on Earth, then we have only described and named about 25% of life forms on the planet. If the 1 trillion figure is correct, then we have done an abysmally poor job, with 99.99% of species still awaiting description.
It’s clear our planet is absolutely teeming with life, even if we cannot yet put a number to the multitudes. The question now is how much of that awe-inspiring diversity we choose to save.
Simon Torok, University of Melbourne; Colleen Boyle, RMIT University; Jenny Gray, University of Melbourne; Julie Arblaster, Monash University; Lynette Bettio, Australian Bureau of Meteorology; Rachel Webster, University of Melbourne, and Ruth Morgan, Monash University
December 24 is the 50th anniversary of Earthrise, arguably one of the most profound images in the history of human culture. When astronaut William Anders photographed a fragile blue sphere set in dark space peeking over the Moon, it changed our perception of our place in space and fuelled environmental awareness around the world.
The photo let us see our planet from a great distance for the first time. The living Earth, surrounded by the darkness of space, appears fragile and vulnerable, with finite resources.
Viewing a small blue Earth against the black backdrop of space, with the barren moonscape in the foreground, evokes feelings of vastness: we are a small planet, orbiting an ordinary star, in an unremarkable galaxy among the billions we can observe. The image prompts emotions of insignificance – Earth is only special because it’s the planet we live on.
As astronaut Jim Lovell said during the live broadcast from Apollo 8, “The vast loneliness is awe-inspiring, and it makes you realise just what you have back there on Earth.”
Earthrise is a testament to the extraordinary capacity of human perception. Although, in 1968, the photograph seemed revelatory and unexpected, it belongs to an extraordinary history of representing the Earth from above. Anders may have produced an image that radically shifted our view of ourselves, but we were ready to see it.
A history of flight
People have always dreamed of flying. As we grew from hot-air balloons to space shuttles, the camera has been there for much of the ride.
After WWII, the US military used captured V-2 rockets to launch motion-picture cameras out of the atmosphere, producing the first images of Earth from space.
Russia’s Sputnik spurred the United States to launch a series of satellites — watching the enemy and the weather — and then NASA turned its attention to the Moon, launching a series of exploratory probes. One (Lunar Orbiter I, 1966) turned its camera across a sliver of the Moon’s surface and found the Earth, rising above it.
Despite not being the “first” image of the Earth from our Moon, Earthrise is special. It was directly witnessed by the astronauts as well as being captured by the camera. It elegantly illustrates how human perception is something that is constantly evolving, often hand in hand with technology.
Earthrise showed us that Earth is a connected system, and any changes made to this system potentially affect the whole of the planet. Although the Apollo missions sought to reveal the Moon, they also powerfully revealed the limits of our own planet. The idea of a Spaceship Earth, with its interdependent ecologies and finite resources, became an icon of a growing environmental movement concerned with the ecological impacts of industrialisation and population growth.
From space, we observe the thin shield provided by our atmosphere, allowing life to flourish on the surface of our planet. Lifeforms created Earth’s atmosphere by removing carbon dioxide and generating free oxygen. They created an unusual mix of gases compared to other planets – an atmosphere with a protective ozone layer and a mix of gases that trap heat and moderate extremes of temperature. Over millions of years, this special mix has allowed a huge diversity of life forms to evolve, including (relatively recently on this time scale) Homo sapiens.
The field of meteorology has benefited enormously from the technology foreshadowed by the Earthrise photo. Our knowledge is no longer limited to Earth-based weather-observing stations.
Satellites can now bring us an Earthrise-type image every ten minutes, allowing us to observe extremes such as tropical cyclones as they form over the ocean, potentially affecting life and land. Importantly, we now possess a long enough record of satellite information so that in many instances we can begin to examine long-term changes of such events.
The human population has doubled in the 50 years since the Earthrise image, resulting in habitat destruction, the spread of pest species and wildfires spurred by climate warming. Every year, our actions endanger more species.
Earth’s climate has undergone enormous changes in the five decades since the Earthrise photo was taken. Much of the increase in Australian and global temperatures has happened in the past 50 years. This warming is affecting us now, with an increase in the frequency of extreme events such as heatwaves, and vast changes across the oceans and polar caps.
With further warming projected, it is important that we take this chance to look back at the Earthrise photo of our little planet, so starkly presented against the vastness of space. The perspective that it offers us can help us choose the path for our planet for the next 50 years.
It reminds us of the wonders of the Earth system, its beauty and its fragility. It encourages us to continue to seek understanding of its weather systems, blue ocean and ice caps through scientific endeavour and sustained monitoring.
The beauty of our planet as seen from afar – and up close – can inspire us to make changes to secure the amazing and diverse animals that share our Earth.
Zoos become conservation organisations, holding, breeding and releasing critically endangered animals. Scientists teach us about the capacities of animals and the threats to their survival.
Communities rise to the challenge and people in their thousands take actions to help wildlife, from buying toilet paper made from recycled paper to not releasing balloons outdoors. If we stand together we can secure a future for all nature on this remarkable planet.
But is a 50-year-old photo enough to reignite the environmental awareness and action required to tackle today’s threats to nature? What will be this generation’s Earthrise moment?
The authors would like to acknowledge the significant contribution of Alicia Sometimes to this article.
Simon Torok, Honorary Fellow, School of Earth Sciences, University of Melbourne; Colleen Boyle, Senior Advisor, Learning and Teaching, RMIT University; Jenny Gray, Chief Executive Officer – Zoos Victoria, University of Melbourne; Julie Arblaster, Associate Professor, Monash University; Lynette Bettio, , Australian Bureau of Meteorology; Rachel Webster, Professor of Physics, University of Melbourne, and Ruth Morgan, Senior Research Fellow, Monash University
Globally, one-third of food produced for human consumption is wasted. Food waste costs Australia A$20 billion each year and is damaging our planet’s resources by contributing to climate change and inefficient land, fertiliser and freshwater use.
And it’s estimated if no further action is taken to slow rising obesity rates, it will cost Australia A$87.7 billion over the next ten years. Preventable chronic diseases are Australia’s leading cause of ill health, and conditions such as coronary heart disease, stroke, high blood pressure, some forms of cancer and type 2 diabetes are linked to obesity and unhealthy diets.
But we can tackle these two major issues of obesity and food waste together.
Avoid over-consumption of food
Described as metabolic food waste, the consumption of food in excess of nutritional requirements uses valuable food system resources and manifests as overweight and obesity.
The first of the Australian dietary guidelines is:
To achieve and maintain a healthy weight, be physically active and choose amounts of nutritious food and drinks to meet your energy needs.
In 2013, researchers defined three principles for a healthy and sustainable diet. The first was:
Any food that is consumed above a person’s energy requirement represents an avoidable environmental burden in the form of greenhouse gas emissions, use of natural resources and pressure on biodiversity.
Reduce consumption of processed, packaged foods
Ultra-processed foods are not only promoting obesity, they pose a great threat to our environment. The damage to our planet not only lies in the manufacture and distribution of these foods but also in their disposal. Food packaging (bottles, containers, wrappers) accounts for almost two-thirds of total packaging waste by volume.
Ultra-processed foods are high in calories, refined sugar, saturated fat and salt, and they’re dominating Australia’s food supply. These products are formulated and marketed to promote over-consumption, contributing to our obesity epidemic.
Healthy and sustainable dietary recommendations promote the consumption of fewer processed foods, which are energy-dense, highly processed and packaged. This ultimately reduces both the risk of dietary imbalances and the unnecessary use of environmental resources.
Author Michael Pollan put it best when he said, “Don’t eat anything your great-grandmother wouldn’t recognise as food.”
So what do we need to do?
In response to the financial and environmental burden of food waste, the federal government’s National Food Waste Strategy aims to halve food waste in Australia by 2030. A$133 million has been allocated over the next decade to a research centre which can assist the environment, public health and economic sectors to work together to address both food waste and obesity.
One of Brazil’s five guiding principles states that dietary recommendations must take into account the impact of the means of production and distribution on social justice and the environment. The Qatar national dietary guidelines explicitly state “reduce leftovers and waste”.
Many would be surprised to learn Australia’s dietary guidelines include tips to minimise food waste:
store food appropriately, dispose of food waste appropriately (e.g. compost, worm farms), keep food safely and select foods with appropriate packaging and recycle.
These recommendations are hidden in Appendix G of our guidelines, despite efforts from leading advocates to give them a more prominent position. To follow international precedence, these recommendations should be moved to a prominent location in our guidelines.
At a local government level, councils can encourage responsible practices to minimise food waste by subsidising worm farms and compost bins, arranging kerbside collection of food scraps and enabling better access to soft plastic recycling programs such as Red Cycle.
Portion and serving sizes should be considered by commercial food settings. Every year Australians eat 2.5 billion meals out and waste 2.2 million tonnes of food via the commercial and industrial sectors. Evidence shows reducing portion sizes in food service settings leads to a reduction in both plate waste and over-consumption.
Given the cost of food waste and obesity to the economy, and the impact on the health of our people and our planet, reducing food waste can address two major problems facing humanity today.
Editor’s note: Curbing damage to Earth’s protective ozone layer is widely viewed as one of the most important successes of the modern environmental era. Earlier this year, however, a study reported that ozone concentrations in the lower level of the stratosphere had been falling since the late 1990s – even though the Montreal Protocol, a global treaty to phase out ozone-depleting chemicals, had been in effect since 1989. This raised questions about whether and how human activities could still be damaging the ozone layer. Atmospheric chemist A.R. Ravishankara, who co-chaired a United Nations/World Meteorological Organization Scientific Assessment panel on stratospheric ozone from 2007 to 2015, provides perspective.
What’s the prevailing view among atmospheric scientists today about the state of the ozone layer?
The overall picture is clear: The Montreal Protocol reduced use of ozone-depleting chemicals and will lead to healing of the ozone layer. This is an important goal because stratospheric ozone protects us from exposure to ultraviolet radiation, which can increase the risk of cataracts, skin cancer and other detrimental effects.
Of course, this forecast would be wrong if nations deviate from their treaty commitments, or if the scientific community fails to detect possible emissions of gases that could deplete the ozone layer but are not covered by the treaty.
Our understanding of stratospheric ozone depletion has grown steadily since the mid-1970s, when Mario Molina and F. Sherwood Rowland first suggested that the ozone layer could be depleted by chlorofluorocarbons, or CFCs – research that earned a Nobel Prize. In 1985 Joseph Farman reported on the formation of an “ozone hole” – actually, a large-scale thinning of the ozone layer – that develops over Antarctica every austral spring.
Further work by Susan Solomon and colleagues clearly attributed the ozone hole to CFCs and other ozone-depleting chemicals that contained the elements chlorine and bromine. They also highlighted unusual reactions that take place on polar stratospheric clouds.
In 1987, the United States and 45 other countries signed the Montreal Protocol, which required them to phase out use of ozone-depleting substances. Today 197 nations have ratified the treaty, which has prevented large-scale ozone layer depletion and its harmful consequences. Ozone-depleting substances are also greenhouse gases that trap heat in the atmosphere, so phasing them out under the Montreal Protocol has also helped to slow climate change.
It takes decades to cleanse CFCs and other ozone depleting substances from the atmosphere, so even after the Montreal Protocol went into effect, their concentrations did not peak until around 1998 and are still high. Nonetheless, based on atmospheric observations, laboratory studies of chemical reactions and numerical models of the stratosphere, there is general consensus among scientists that the ozone layer is on track to recover around 2060, give or take a decade. We also know that the future of the ozone layer is intricately intertwined with climate change.
What could explain the continued decline in ozone in the lower stratosphere that was reported earlier this year?
Of course, there are still some gaps in our knowledge of the ozone layer, and these two new reports have spotlighted such gaps.
The first study reported that although ozone concentrations were increasing in the upper stratosphere, they were still declining in the lower stratosphere. It suggested several possible causes, such as increases in uncontrolled, very short-lived gases produced from human activities that can deplete the ozone layer, as well as changes in atmospheric circulation due to climate change.
The second study identified rising levels of certain chlorinated chemicals, referred to as very short-lived substances, that could continue to deplete the ozone layer.
These reports were a little surprising, but not shocking. Scientists expect that we will continue to add to our knowledge of ozone layer, and our understanding will emerge as we digest these findings. The Montreal Protocol requires the scientific community to carry out scientific assessments of ozone depletion every four years precisely because we expect that new information like this will continue to emerge. One of those reviews is under way now and will be published later this year.
If industrial activities causing this decline in the lower stratosphere, Montreal Protocol member countries can amend the treaty to address these new threats. They did so in 2016 to phase out hydrofluorocarbons – coolants, used in air conditioners and refrigerators, that were found to be powerful greenhouse gases.
If ozone levels in the lower stratosphere have been decreasing for 20 years, why are scientists just detecting that trend now?
Ozone levels change naturally from year to year, so scientists need to look at data over long time periods to tease out trends. The potential for short-lived chlorine and bromine gases to affect the ozone layer has been recognized for a while. More recently, scientists have been measuring concentrations in the atmosphere of dichloromethane, a liquid that is widely used as a solvent, and deduced that if it continues to increase, it will be a potential problem. Trends in emissions of these compounds are uncertain, but some recent results suggest that they are not increasing as rapidly as they appeared to be a few years ago.
Did these studies find any changes in the ozone hole?
No, they did not. They examined ozone changes within 60 degrees of the equator, not over the Antarctic. We do not expect very short-lived substance emissions to significantly influence the ozone hole unless they increase drastically, but this is one more reason to keep an eye on them.
Do these recent findings make you question whether the Montreal Protocol is effective?
No! Indeed, they strengthen my trust in it.
With any environmental agreement, whether it addresses ozone depletion, acid rain, climate change or other issues, it is important to be vigilant during the “accountability phase” – the period after policy decisions have been made and before the targeted results are expected. I am confident that if scientific findings warrant it, Montreal Protocol countries will take further action, and that my granddaughters will see the day when we eliminate the ozone hole.