Global Warming
[The distinction between weather and climate is an important one. For example, weather forecasts can be fairly specific but are little use more than a few days into the future.]
Ql. What exactly is the climate?
Climate, like weather, describes the state of the atmosphere in terms of factors such as temperature, wind and rainfall. But whereas weather describes conditions as measured in hours, days or weeks, the climate is average weather conditions measured over the longer term: months, years or decades.
The distinction between weather and climate is an important one. For example, weather forecasts can be fairly specific but are little use more than a few days into the future. By contrast, climate predictions focus on expected changes in average conditions, while recognizing that individual days, weeks, months or years will always buck the longer-term trend.
Each area of the world has its own climate, though scientists also study the planet's overall climate system. The term climate comes from the Greek klinein, or "slope", describing how the angle that the sun hits the earth varies in different regions.
Q2. What is climate change?
Any process that causes adjustments to a climate system - from a volcanic eruption to a cyclical change in solar activity - is creating "climate change". Today, however, the phrase is often used as shorthand for anthropogenic climate change i.e. climate change caused by humans. The principal way in which humans are understood to be affecting the climate is through the release of heat-trapping greenhouse gases into the air. Climate change is used interchangeably with "global warming", reflecting the strong warming trend that scientists have observed over the past century or so.
Q3. Is the world really getting warmer?
Yes. There is agreement among the scientists that the earth has warmed in the last century. Here's how the world's most prestigious scientific bodies put it in a joint statement signed by the heads of the national science academies in Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, the UK and the US: "Climate change is real. There will always be uncertainty in understanding a system as complex as the world's climate. However, there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems."
In other words, we know that the planet is warming because temperature measurements show it and because these measurements are borne out by observations such as rising sea levels, retreating snow cover and glaciers, longer growing seasons and shifting wildlife. There's a good summary of ten different warming indicators - all of which support the theory that temperatures are increasing - in this report by the US National Oceanic and Atmospheric Administration.
The same academies listed above - which represent the definitive voice of science in each country - have restated their position on global warming a number of times since that original statement was issued in 2005. The most recent joint statement from 2009 said that "climate change is happening even faster than previously estimated".
The IPCC, a UN scientific body convened to assess and surmise science relating to climate change, stated in its last assessment of the evidence that "warming of the climate system is unequivocal... The same report - drawing on the full range of published science papers on the subject - points to a rise of about three-quarters of a degree celsius in the past century, with much of that warming taking place over the past few decades.
Of course, the fact that the world is warming doesn't mean that it is getting hotter in a uniform way. The long-term rise in temperature is affected by shorter term factors such as changes in solar activity and regional cycles such as the oscillation between El Nino and La Nina conditions in the Pacific. And some regions - such as the Arctic - are warming significantly faster than others.
Taken as a whole, however, and averaged out over decades to remove year-to-year variability, the warming trend is clear.
Q4. Are humans definitely causing global warming?
The world's most respected scientific bodies have stated that there is strong evidence that humans are driving the warming. The 2005 joint statement from the national academies of Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, the UK and the US said:
"It is likely that most of the warming in recent decades can be attributed to human activities."
Countless more recent statements and reports from the world's leading scientific bodies have said the same thing. For example, a 2010 summary of climate science by the Royal Society stated that:
"There is strong evidence that the warming of the Earth over the last half-century has been caused largely by human activity, such as the burning of fossil fuels and changes in land use, including agriculture and deforestation."
There is strong evidence that the warming of the Earth over the last half-century has been caused largely by human activity, such as the burning of fossil fuels and changes in land use, including agriculture and deforestation."
The idea that humans could change the planet's climate may be counter-intuitive, but the basic science is well understood.
Each year, human activity causes billions of tonnes of greenhouse gases to be released into the atmosphere. These gases capture heat that would otherwise escape to space - the equivalent of wrapping the planet in an invisible blanket.
Of course, the planet's climate has always been in flux thanks to "natural" factors such as changes in solar or volcanic activity, or cycles relating the Earth's orbit around the sun. According to the scientific literature, however, the warming recorded to date matches the pattern of warming we would expect from a build-up of greenhouse gas in the atmosphere.
The only way to prove with 100% certainty that humans are responsible for global warming would be to run an experiment with two identical Earths - one with human influence and one without. That obviously is not possible, and so most scientists are careful not to state human influence as an absolute certainty.