Man-made Change
Voices from Brazil:
"The forest has to be acknowledged by the politics, because it has an important role in keeping
balance. Not only climate balance, but also to create and support a new management model for the planet."
Almir Surui, Indigenous Leader from the Suruí People
But couldn’t the warming still be a natural variation—perhaps something like a change in the sun’s output?
Scientists have looked for alternative explanations based on the natural causes of climate changes in Earth’s past. None of them explain what we are seeing.
Reconstruction of Northern Hemisphere temperatures.
They’ve also looked at what has been normal temperature variation for the past few thousand years. The temperatures we’re seeing now are outside what’s been “normal” during that time.
CO2 model temperature estimates with (blue) and without (pink) anthropogenic climate influences compared to observations (black line).
Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Figure 2.5, Geneva, Switzerland.
Computer models help make the case for a human cause. Scientists use the models to simulate climate conditions over the past 100 years. By putting in and leaving out major factors that change climate, they can see how these factors affect global temperature. They can then compare the model estimates to the temperatures actually observed over the years.
When only natural climate factors are put into the models, the results indicate that things like solar and volcanic conditions would likely have produced cooling in the last 50 years.
But when scientists add the greenhouse gases and particles from human activities, the model results correspond very well to the warming that we’ve seen over the past century.