Another sign of Graves' disease is hyperthyroidism, i.e., overproduction of the thyroid hormones T3 and T4. Normal thyroid levels are also seen, and occasionally also hypothyroidism, which may assist in causing goitre (though it is not the cause of the Graves' disease).
Without this greenhouse effect, the average surface temperature would be 255 degrees kelvin (-18 degrees Celsius or 0 degrees Fahrenheit); a temperature so low that all water on Earth would freeze, the oceans would turn into ice and life, as we know it, would not exist.
By 2020, models project that global surface temperature will be more than 0.5Â°C (0.9Â°F) warmer than the 1986-2005 average, regardless of which carbon dioxide emissions pathway the world follows.