Climate change refers to changes in global or regional climate determined over a long term - typically a minimum of 30 years. Individual short-term weather events such as an intense storm or heat wave are a normal aspect of weather variability, and are not a sign of climate change. Even several weeks of unusual weather conditions are not sufficient to prove that climate is changing. Evidence of a changing climate is found by looking at long-term averages of atmospheric data, over 30 years or more. When we see changes in atmospheric conditions over these long term intervals, we see evidence of a changing climate.
While weather can change very quickly, in hours (and sometimes minutes!), climate changes over longer time frames depending on changes in the factors that determine Earth's climate. Earth's climate changes when there are changes in the energy received at Earth from the Sun and when there are changes in the amount of energy that is held in the Earth system. Changes in these factors occur over long-term intervals - decades or more. There are numerous ways in which these factors can change over the long term - ranging from changes in the amount of greenhouse gases in the atmosphere to small variations in the tilt of the Earth's axis - and Earth history demonstrates multiple intervals when our climate was very different.
Because long-term weather conditions effect the type of plants and animals that live in regions on the planet, changes in climate can have significant effects on many aspects of life on the planet. In addition to changing atmospheric temperature, changing climate influences precipitation, winds, the intensity of storms, the persistence of ice, sea level, the temperature of water bodies, the distribution and survival of plant and animal species, and the distribution of disease. These, in turn, can have dramatic impacts on how our society functions.
Today, thousands of climate scientists around the world are studying how the Earth's climate is changing. This large scientific community has been studying Earth's climate for decades, thanks to advances in scientific understanding, computing, and modelling. However, climate science is not new - over one hundred years ago scientists recognized that humans could influence the Earth's climate through the growth of greenhouse gases in the atmosphere.
The scientific consensus today is that Earth's climate is changing, and that the cause of the change is the accumulation of greenhouse gases in the atmosphere produced mainly by the consumption of fossil fuels by humans to power our society. A wider community of specialists studies the potential impact of changing climate on Earth's ecosystems, economy, and society. Climate models are an important tool which can be used to examine future climate scenarios, based on assumed levels of greenhouse gas production driven by our consumption choices.