It has come to my attention that there is some discussion about the reality of Global Warming. Some people don't believe in Global Warming, other people say that Global Warming exists, but is not man-induced. This topic will hopefully show you how Earth scientists view Global Warming. I will keep this as short and simple as possible, so I will not venture into the chemistry, physics and mathematics behind certain processes. To help me explain, I will first introduce some technical terms. Climate: The average weather in a region over a period of 30 years. The region we are referring to here is Earth. Climate Change: The (naturally occuring) change in Climate. Greenhouse effect: When sunlight hits the Earth's surface, it warms the surface. The Earth beams out this heat into space. Certain gases, like water vapor, carbon dioxide and methane, absorb this heat, trapping it in the atmosphere, thus creating a habitable zone. these gases are called greenhouse gases. Global Warming: The idea that increased greenhouse gases cause the Earth's temperature to rise globally. Let's begin: The most important question is, of course, the obvious one: Is the climate really changing? The answer to this is easy: Yes. 11,700 years ago, at the end of the Pleistocene epoch, the climate became gradually warmer, which caused the land ice masses to melt. In the past 12,000 years the seawater level has risen to current level. The projection is that it will continue to rise for another thousand years or so. Furthermore, in the Cretaceous period, the sea level was projected to be about 100 meters higher than present levels. So yes, climates can change. It is generally accepted in the scientific world that climates change. The second question is then, logically, what causes Climate Change? This isn't as easy a question to answer as the last. Climate Change is caused by a variety of different effects all correlating between eachother, like the carbon cycle, the intensity of the sun, the Milankovich curve and the amount of certain greenhouse gases in the atmosphere. Like a row of dominoes, when one changes, the others react to this change (except for the Sun, of course). This is a rather delicate system which is never truly in balance, thus causing changing climates. This delicate "balance" means that it can be easily this is where we Humans come in, and were it gets really difficult to tell facts from opinions, and truths from fictions. So, the big question: Are humans changing the climate? Unfortunately, we are. Although there is no consensus on the exact how and what of Global Warming, it is generally accepted that human activity is affecting the Earths climate. The easiest example is that of the City Climate. In a big city, the average temperature is higher that on the countryside. This has to do with i.a. dust particles from cars that absorb heat and act as greenhouse gas, and the lack of vegetation. On a global scale, the climate has changed, and is expected to change, because of the emission of greenhouse gases, CO2 in particular. This is considered a fact in the scientific world. The real discussions concern the speed and intensity of human Climate Change, and this is were the difficulties come from. When two scientist disagree about Climate Change, they disagree about the speed and intensity, not the existence. The media, however, take a completely different stance on the matter, and report the incident like a disagreement on the existence on Climate Change. That's a big bite to swallow, and it needs some scientific spices: Proof. What proof is there that the climate has changed, and will continue to change over the years to come, because of human activity? First, there is the extreme increase of atmospheric CO2 since the 19th century, or the Industrial Revolution, to be precise. Atmospheric samples from all over the world have been compared to "prehistorical atmospheres" from gas bubbles in Ice Cores. Current CO2 levels, which are at 300-400 parts per million (Berendsen, 2008), are almost 40% higher that in previous periods (Houghton, 2009). It is suffice to say that this has an effect on the climate. Finally, I will explain where the disagreements within the scientific world come from. Us scientist (or scientists in training) don't know exactly how fast the climate is changing. This has to do with a lot of difficult factors, like the ability of the oceans to absorb CO2 from the atmosphere or the release of methane from thawing permafrost areas. Therefore, scientists disagree on the speed of global change. For more information, send me a PM, and I'll try to give you an answer. The real science-y bit: sources : Houghton, J. (2009). [i]Global Warming: The complete briefing.[/i] Cambrigde University Press, Cambridge. Berendsen, H.J.A. (2008) [i]Landschap in delen: Overzicht van de geofactoren[/i]. Koninklijke van Gorcum, Assen. For those with cash to spare I would suggest you buy the book Global Warming.