I’ve been chopping down some old man pine for firewood. The trees are getting on for a hundred years old, probably planted around the same time as our farmhouse was built. They’ve done a great job providing shelter, but their shade is not welcome on a corner of our young vineyard and their roots are out-competing the vines. As the gnarly old trunks are chainsawed into rounds for splitting, the slabs of wood reveal the progression of the trees’ lives. Wide rings mark years of rapid growth, narrow ones mark years when the tree struggled. They show warm years, cold years, wet years and dry years. They sum up the weather, integrate it, and present the information to be read by those with the key.

Human memory is much more fallible. If I asked the families who’ve lived here about the weather they experienced over the last century, they’d remember nor’west gales that uprooted trees, the great snowfall of 1945, big floods and testing droughts, heavy rain that brought landslips and gullying that ate into paddocks. They’d remember warm summers and cold winters, and perhaps be able to show me rainfall records. But if I were to ask them how the climate had changed, they would be unreliable witnesses. We remember the big things, but we’re not designed to record subtle changes in long-term averages.

Climate change is about variations in small numbers, and the power of small numbers is something we underestimate at our peril. Carbon dioxide, the greenhouse gas that’s getting us into trouble, only amounts to about 385 parts per million of the gases in the atmosphere—not a large amount. But that’s a third more CO2 than 150 years ago, enough to have caused, and continue to cause, significant changes in local and global climate. Twelve of the 13 years from 1995 to 2007 have been the warmest since 1850, while 1998 and 2005 are the warmest years since records began. Over the last 100 years, New Zealand’s temperature has increased by 0.9ºC.