It is a demonstrated scientific fact that the presence of CO2, together with methane, nitrous oxide and fluorocarbons in the atmosphere have a greenhouse effect. This is caused by the ability of these gases to trap solar radiation and warm the troposphere. A certain amount of solar radiation is absorbed by land, air and seawater with most of the balance being reflected back into space, particularly from surfaces covered by snow and ice.
Greenhouse gases capture solar energy and radiate it back on to the surface of the earth. The more of these gases present, the greater the amount of radiation and the warmer the surface of the planet becomes. CO2 is the most common of the greenhouse gases. We and most other life forms can not live with too much or too little in the atmosphere. There is a fine balance in the quantity present which makes most of the planet habitable by animal and other life forms.
Too little, less than about 330 parts per million (ppm) and too little solar energy is trapped in the atmosphere. The surface of the planet becomes too cold for homo sapiens and the life forms on which they depend to survive. Too much, more than 430ppm and much of the planets surface becomes too warm and the atmosphere too laden with moisture for us to survive.
Given time, animal and other life forms are often able to adapt to warmer or cooler conditions but when there is relatively rapid movement in the concentration of CO2 in the atmosphere outside the range of say 300-450ppm, adaption is difficult or impossible. Species which have had 100,000 years or more to adapt to and thrive in a stable environment or one which changes very slowly, often find sudden change very difficult to cope with. They are faced with the stark choice, adapt or face extinction.
It is not the presence of a particular concentration of CO2 per se, which results in this choice but, all importantly, the speed with which it changes. Let there be no doubt that the speed with which CO2 concentration in the atmosphere is now rising is very rapid indeed, much faster than in previous interglacial periods. Its effects are so profound, so far reaching, that humans and the species on which they depend will find it very difficult to adapt. They are unlikely to be able to do so in their present rapidly increasing numbers.
What is causing increased concentration of CO2 and other greenhouse gases in the atmosphere? The answer is very simple. Primarily, we are. There are two ways of increasing the presence of CO2 in the atmosphere: First, by reducing the concentration of other gases such as oxygen or nitrogen relative to CO2 content. Second and much more significantly, by adding CO2 to the atmosphere by burning fossil fuels. Both occur as a result of human activity.
A rapidly increasing human population feeds itself by producing even greater numbers of domestic animals such as cattle, sheep and pigs. Combined, the burgeoning number of animals, vastly in excess of the pre-industrial number, are breathing in and consuming air, retaining and so reducing oxygen while exhaling CO2 back into the atmosphere. The effect is exacerbated by the related human activity of land clearing for food and energy production, reducing the number of trees taking up CO2 from the atmosphere.
The weight of CO2 produced from burning given quantities of fossil fuels such as coal, gas, and oil products is known. The quantity of those fuels used to produce the energy needed to sustain or increase human activities is also known. From this information we can quantify the magnitude of greenhouse gases added to the atmosphere per annum. We can and we do measure the concentration of CO2 expressed as ppm in the atmosphere - and find that it is continually rising.
A further cause for concern is that land in the northern hemisphere which has been frozen for millennia will thaw due to global warming. As this occurs, decaying vegetation and animal remains will resume decomposing and emit significant quantities of greenhouse gases, including methane, a particularly potent contributor to the greenhouse effect. This is one of several reasons for wanting to limit global warming by 2100 to no more than 2C above the pre-industrial era.
Some, like Ian Plimer, argue that increased CO2 in the atmosphere has little if anything to do with human activity. They claim that CO2 is almost entirely produced naturally, primarily by volcanoes. The only problem with this contention is that it is quite unsupported by evidence. If volcanic activity were responsible for a significant increase in CO2, we would expect to detect such increases, particularly following major eruptions. But we do not.
They claim that increasing CO2 in the atmosphere is a good thing since it promotes plant growth - assuming that plants can be grown in changing climatic conditions leaving vast areas of cropland arid or with insufficient water. They ignore the fact that increased absorption of CO2 by seawater results in acidification causing massive damage to the marine ecology. This includes destruction of coral reefs, jelly fish infestations and the threat of a serious break in the marine food chain.
These are not imaginary developments. They are already happening because of climate changes caused by global warming resulting from the rising presence of greenhouse gases in the atmosphere.
Others claim that even if atmospheric CO2 is increasing, it does not have a greenhouse effect, is not responsible for global warming or consequential climate change. Here again they offer no supporting evidence, though in an effort to “prove” their view, proponents are willing to and frequently do put forward “evidence” which has clearly been forged, can not be independently verified or is demonstrably wrong. For example, it is claimed that land based snow, glaciers and Arctic sea ice are not diminishing but are in fact growing, even though empirical evidence shows them to be contracting.
The less rational simply assert that there is no science to support claims of increased CO2 emissions or its greenhouse effects. They are convinced that by simply denying the existence of a scientific basis for them, that denial is, by itself, proof of their claim. Some even declare that calls for reduction of emissions are nothing more than a wicked international left-wing conspiracy led by the IPCC or corrupt scientists in a bid to impose world government or new taxes.
Arguing against the irrational is difficult, particularly when its proponents either fail or refuse to offer a shred of cogent evidence to support their position - primarily because they have no evidence. Fortunately the vast majority of people, including every national government in the world, have more sense and accept the scientific explanation for global warming. They do so because it is supported by science and empirical measurements, which provide a persuasive explanation for the causes of global warming and, importantly, how to deal with them.