says who? the earth has been around for how long? it has gone through much much worse warming periods before humans existed.
The sun has remained more or less constant throughout the course of history. Nasa has good data on the sun, we know this. So it can't be the sun. In order for the Sun to be responsible for the recent warming, there would need to be a long-term increase in the solar constant over the past few decades. The measurements show no evidence of this. Thus, we can conclude with high confidence that the rapid warming of the past few decades is not caused by a brightening of the Sun.
Continent's moving could also have an effect on the climate, but this occurs to slowly to explain the sudden rise of temps we are experiencing. This is good to explain the slow change of temps over extended periods of time, but not quick ones. It takes millions of years for continental movement to cause significant climate change.
The Earth's orbit also has an effect on climate. These orbital variations are so slow that it takes at least thousands of years to make any significant change in the amount of or distribution of incoming sunlight. The warming of the past century has been much too fast to be caused by these slow orbital variations. The warming must be due to other causes.
Internal variablities in our climate such as the El Nino and La Nina effect our climate by warming and cooling it but they occur every few years and last a year or so. No mechanism of internal variability can explain the warming. Ultimately, we cannot exclude internal variability. However, there is no evidence supporting it, either.
Then there are Green House gases.
The geologic record over the past 500 million years shows a strong correlation between temperature and atmospheric carbon dioxide. Widespread ice existed when carbon dioxide was low, and no ice was found when carbon dioxide was high. The correlation between greenhouse gases and temperature is particularly clear during an event roughly 55 million years ago known as the Paleocene–Eocene Thermal Maximum or PETM. This event began with a massive release of either carbon dioxide or methane. Which in turn led to an increase in the Earth’s global average temperature of 5–9°C over the next few thousand years. The amount of carbon was so immense that when the CO2 dissolved into the oceans, the oceans became significantly more acidic. This in turn dissolved calcium carbonate in the sediments at the bottom of the ocean.
The temperatures remained elevated for 100,000 years or so, which is about the length of time it takes the carbon cycle to fully remove the carbon from the atmosphere. The amount of carbon released, a few thousand gigatonnes of carbon, is comparable with the amount contained in all of the Earth’s fossil fuels.
Thus, the PETM is sometimes viewed as a good analog to what will happen if humans burn all of the fossil fuels over the next few centuries. One important difference, however, is that humans are on a pace to release the carbon over several hundred years, whereas it was released during the PETM over several thousand years.Thus, we can expect even more rapid warming than that experienced during the PETM, a period of very rapid warming.
