TEMPERATURES were warmer in the northern hemisphere 1000 and 2000 years ago, well before the industrial revolution when the level of carbon dioxide in the atmosphere was 40 per cent lower than it is today.
The authors of a report, published in Global and Planetary Change, said findings that the northern hemisphere was warmer during both the Roman period, 2000 years ago, and medieval warming, 1000 years ago, highlighted uncertainties in making climate predictions.
The paper does not seek to disprove contemporary climate change nor comment on the relationship between atmospheric carbon dioxide and global temperatures. But it rejects claims that the historic global temperature record is well understood, and highlights difficulties in modelling future climate scenarios.
The report concludes the temperature history of the past 1000 years is much less well understood than has often been claimed.
It says the pre-1400 AD temperature variance at a hemisphere level is largely unknown. "Expert teams were needed to assess existing records and to reduce uncertainties associated with millennium-length temperature reconstructions, before we can usefully constrain future climate scenarios," the paper says.
David Jones, head of climate monitoring and prediction services at Australian Bureau of Meteorology, said historic warming during the Roman and medieval period could be explained by changes in solar activity.
But he said it was difficult to use the temperature record from one location as a guide to what was happening to global temperatures.