A greenhouse gas is a gas that absorbs and emits radiant energy. Greenhouse gases cause the greenhouse effect which is a warming of Earth’s land, air and oceans. The primary greenhouse gases in Earth's atmosphere are water vapour, carbon dioxide, methane, nitrous oxide and ozone.

Greenhouse gas (GHG) concentration is measured in parts per million of Earth’s atmosphere. Larger emissions of greenhouse gases lead to higher concentrations in the atmosphere. Global atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and certain manufactured greenhouse gases have all risen significantly over the last few hundred years. For the 800,000 years prior to Earth’s recent industrial revolution the concentration of carbon dioxide (CO₂)— which comprises about 83% of the GHGs—was between 200 and 300 parts per million (ppm) and was mainly less than 250 ppm. The concentration of CO₂ began rising with the onset of the Industrial Revolution at which time humans began producing energy primarily through burning fossil fuels (mainly coal and oil). By 1950 the concentration of CO₂ was over 300 ppm and rising steadily. In May 2019 CO₂ concentration was just above 415ppm. The result is a measurably warmer planet, with increasing occurrences of extremes of weather (droughts, storms, heatwaves) and related events (famines, floods, fires).

What are the likely effects of 450ppm of CO₂ by 2040?

A noticeably warmer planet, with further extremes and turbulence in the weather, is the most obvious result. There will be a growing recognition that attempts to reduce GHGs (and thus to limit increasing temperatures) have been too little too late. Governments will continue to come under pressure from citizenry to close down production of energy from fossil fuels and instead increase renewable energy production. The social and political tensions attending these changes may be expressed in undesirable, although understandable, ways. Direct action, including civil disobedience, will become more likely as frustration at inadequate governmental and corporate response produces anger and impatience. Government repression and control will probably become more common. The continuing emergence of fascistic political movements could be a major danger.

A slowing of human population increase may result from increased deaths of infants, the elderly and other people made vulnerable through illness, malnutrition, starvation and heat-related stresses. Humanitarian rescue will be stretched and may become inadequate. Scarcity of potable water will become an issue of survival.

Scientific projections of an increased concentration in CO₂ to 660ppm by 2100 will, of course, exacerbate all the above scenarios, plus many more. This is a plausible outcome, although not certain. If by 2100 we are experiencing a 4⁰C increase in average global temperature above current levels, the scenarios described below are possible.

What are the likely effects of 660ppm of CO₂ by 2100?

By this time wealthy people will be occupying underground homes cooled via renewable local power. Outdoor clothing, making use of space travel technology, will be needed for any exposure to planetary surface conditions longer than an hour or two in temperatures over 50⁰C .

For those unable to afford climate-protective housing and clothing, global population will continue to decline. Desperation and depression, in addition to rational motivation, will build on voluntary assisted dying laws to enable those with climate-affected illnesses to escape the worst aspects of extreme climate; and do-it-yourself rational suicide will be commonplace. Cremation will be outlawed and will be replaced by composting of corpses. Women will resist childbearing, even if their health is adequate for it.

In vast areas of the planet there will be persistent crop failures; localised famine will be common. Access to appropriate food will be restricted, not to mention water. People with wealth will ensure they have adequate supplies and, among the less realistic wealthy communities, fantasies of living off-planet will be entertained. Agricultural production will be problematic at best, despite gene manipulation, factory production of artificial meat, and efforts by scientists and industry to create a new green revolution. Animal husbandry will be unable to prevent a permanent decline in farm animals and protein will be in short supply.

Sea level rise by 2100 will be a serious problem, particularly for low lying areas such as many Pacific islands, Bangladesh in the delta areas, and numerous other places including, for example, USA’s Manhattan and Miami, and many coastal areas in Australia. Massive movements of climate refugees will be a challenge, not only for the millions forced to move to higher ground.

Due to increased warming of the oceans, their acidification and human overfishing, fish stocks will collapse with untold consequences for many species, including humans.

The Australian Academy of Science website makes the following observations:

“The impacts of climate change often act to amplify other stresses. For example, many natural ecosystems are already subject to urban encroachment, fragmentation, deforestation, invasive species, introduced pathogens and pressure on water resources. Some societies suffer warfare and civil unrest, overpopulation, poverty and sinking land in high population river deltas. Multiple stresses do not simply add to each other in complex systems like these; rather, they cascade together in unexpected ways. Therefore, climate change impacts, interacting with other stresses, have the potential to shift some ecosystems and societies into new states with significant consequences for human well-being. For moderate levels of climate change, developed countries such as Australia are well placed to manage and adapt to such cascading impacts. However, developing nations, especially the least developed, face risks from projected impacts that may exceed capacities to adapt successfully. As climate change intensifies, especially under high-emission pathways, adaptive capacities may be exceeded even in developed countries.”

Climate change has significant implications for our health. Rising temperatures will likely lead to increased air pollution, a longer and more intense allergy season, the spread of insect-borne diseases, more frequent and dangerous heat waves, and heavier rainstorms and flooding. All of these changes pose serious, and costly, risks to public health.

Climate is changing and the human population is responsible for it. Without a doubt we will live in a very different world as we age, and the world we pass on to future generations will not be a pretty one. We must avoid the worst case of a world in which climate change exceeds 2, or 3, or 4+ degrees Celsius. If we do not avoid it, we humans will be the creators of our own extinction.