Around the world, thousands of scientists have devoted their professional lives to studying the climate. Not centrally organised, they sometimes build temporary affiliations but they remain scientists throughout – that is, they are independent, constantly challenge each other and are committed to searching for truth through objective, independently verifiable evidence.

Overwhelmingly, this evidence has led to four conclusions. The first is that the world is warming. The global average temperature has increased by about 0.8 degrees since 1850, with most of the increase occurring since 1950. The warming varies among decades because of natural fluctuations but the overall trend has been inexorably upward.

Warming is evident in other indicators, such as rising sea levels and reduced sea-ice and snow cover. Of these, the most important measure is the extra heat in the oceans, which is steadily rising. Claims that climate change has reversed since 1998 are misrepresentations of the full data.

The second conclusion is that the dominant cause of the warming since about 1950 is the increase in the atmospheric concentrations of greenhouse gases released by human activities, of which carbon dioxide (CO

This critical conclusion is based on several independent lines of evidence, including basic physics, studies of climate changes in both in the geological past and in the industrial era, and finally – but far from solely – from the predictions of climate models. Together, these provide an overwhelming case that increasing greenhouse gas concentrations cause warming, and that CO