 This graph shows that carbon dioxide levels in the atmosphere are higher now than at any time that humans have inhabited this planet. We can measure the concentration in the past, for example, by analysing tiny pockets of air that have been trapped in Antarctic ice as it got laid down year by year. Global temperatures are difficult to monitor because they are changing all the time, but we can take an average and now that we have satellites we can measure surface temperature of both land and ocean very accurately. It's more difficult to try to work out global temperatures in the past. Human records only began systematically a hundred years ago, but there are many ways we can work out or reconstruct global temperatures stretching back millions of years. One such method measures the ratios of oxygen isotopes found in water that makes up Antarctic ice cores. The more oxygen 18 in the ice, the higher the temperature at which it evaporated from the oceans to fall as snow making up the Antarctic ice sheets. Sea level Using satellites, we are able to measure changes in sea level of just a few centimetres. Previously, with a constant movement of the waves and tides, it was difficult to measure this accurately. If you are familiar with global positioning satellites, that's GPS navigation, you will appreciate that we can make very accurate measurements using satellites. Storms and weather extremes In the first decade of the 21st century, storms became more frequent and more violent. Hurricanes in the Caribbean, typhoons in the China seas and heavier monsoons in India. We also witnessed more droughts and heat waves leading to increased incidents of forest fires in hot countries such as Australia and Greece. Melting ice Glaciers in mountains around the world are melting back. Europe's glaciers are thought to have lost around a quarter of their mass in the first decade of the 21st century. Almost all of Alaska's 2,000 glaciers are set to disappear by the end of the 21st century. Glaciers are important sources of water in the summer or dry season, especially in hot climates. They melt gradually, keeping the rivers flowing. Winter snow replenishes the ice ready for the next season. But currently more ice is melting than is replaced by snow. Perhaps equally dramatic is the disappearance of arctic ice. Each year more ice melts in the summer than reforms in the winter. Some scientists predict that the arctic will be ice-free for a few months in the summer before the end of this century. To summarise, modern technology has allowed us to track rises in global temperatures, sea levels and carbon dioxide levels with a level of accuracy and reliability that was previously unachievable. We are also able to estimate global temperatures from millions of years ago using such methods as isotope ratio analysis. We now have a clear body of evidence, which suggests not only that global warming is taking place, but that it is taking place at an alarming rate. This position is accepted by the United Nations and virtually all countries. It is now time for the world to act.