Key Points:

The world will be hotter than we think if no action is taken to cut greenhouse emissions, a Wellington conference will hear today.

United States climate modelling expert Matthew Huber - who is speaking at the Greenhouse Earth Symposium at Te Papa today - says at least one climate model used by the Intergovernmental Panel on Climate Change produces temperatures that are cooler than the real world. Dr Huber is one of several visiting scientists who use a dramatic period of warming 55 million years ago to predict what will happen in the future.

The associate professor in Earth and Atmospheric Sciences at Purdue University in Indiana tested a respected climate model by applying it to the early Eocene period - a time when greenhouses gases entered the atmosphere at about the same rate they are today.

He found the model - thought to be one of the better ones for predicting climate change - produced temperatures cooler than what really occurred. "When we put in everything that we think is right, the models are never warm enough," he said. "It seems the models are not sensitive enough to greenhouse gases."

Dr Huber said the results meant the model (called the National Centre for Atmospheric Research's community climate system model) was likely to be on the cool side when predicting global warming today.

Scientists believe the earth warmed between 4C and 8C early in the Eocene period, after natural forces released huge amounts of carbon.

Paul Pearson, a professor in paleoclimatology at Cardiff University, who will also speak today, said geological evidence from the Eocene showed warming would be at the upper end of what models predicted.

Studies of sea warming near Tanzania showed surface temperatures reached 34C in the Eocene - 4 degrees higher than what many scientists believed was the limit.

Dr Huber said if greenhouse gas emissions carried on as they were, CO2 levels would be high enough to melt the Greenland and Antarctic ice sheets in 100-200 years, making the sea rise about 100m.

The symposium is part of the three-day Climatic and Biotic Events of the Paleogene conference, which ends in Wellington tomorrow.