evidence: Greenhouse gases - which trap the Sun's heat - are the crucial link between temperature rise and human activities. The most important is carbon dioxide (CO2), because of its abundance in the atmosphere. [...] There's a way we can show definitively where this extra CO2 came from. The carbon produced by burning fossil fuels has a distinctive chemical signature. Tree rings and polar ice both record changes in atmospheric chemistry. When examined they show that carbon - specifically from fossil sources - has risen significantly since 1850.