By Alex Kirby
BBC News Online environment correspondent
An international team is pioneering a new source of information about climate change: old sailing ships' logbooks.
East Indiaman under sail: The data collected was vital
The team, which is led by an expert from the University of Sunderland, UK, is the Climatological Database for the World's Oceans, or Cliwoc for short.
The 19th and 18th Century logbooks from UK, Dutch, French and Spanish fleets yield "consistent and reliable" data.
Cliwoc says its work is slowly building up "one of the most accurate pictures yet of daily weather over the oceans."
Cliwoc says it "aims to discover more about the changing climate over the world's oceans before industrialisation could have had any significant influence on climate and weather."
Three years ago, the team began work on the logbooks, which span the years from 1750 to 1850. After that oceanic weather data depended much more on instrumental measurements.
Before 1750 the range of data available was much more limited geographically, but for the century Cliwoc is concentrating on the researchers say they have "a pretty good global spread, except for the Pacific".
Benchmark from the past
The logs studied include those from voyages made by the explorer Captain James Cook, who was born in the north of England not far from Sunderland.
The team leader, Dr Dennis Wheeler, of the University of Sunderland, said: "A lot of work has been done recently with world meteorological records going back 150 years. Our work goes back much further.
There are far more logbooks with information to yield
"Although oceans cover 75% of the Earth's surface, we had very little information about the weather. These logs help us understand how climate changed in the past, which is a very useful tool when predicting climate change in the future.
"For the first time, with the exception of the Pacific, we can show the daily climate change for all major oceans between 1750 and 1850 and compare it to today's conditions."
The Sunderland team works with colleagues from a range of international organisations, including the UK's National Maritime Museum and University of East Anglia, the University of Madrid, the Royal Dutch Meteorological Institute, and the University of Mendoza in Argentina.
Ships' officers during the years the team has studied needed very accurate records and updated their logs daily, and sometimes hourly.
Cliwoc says there are several reasons why the rich source of information the logbooks offer has been largely ignored till now.
One is that the data were not obtained from instruments, but from human observations and estimates, which it says led some scientists to distrust them.
The records are consistent and reliable
Another is the difficulty of penetrating "the curious style and vocabulary of mariners of those distant times".
And the sheer number of records available, the researchers say, "present a challenge, not of data shortage, but of over-abundance".
But they conclude: "The abundance of data for wind force and direction is invaluable. It tells us much about the broad patterns of atmospheric behaviour related to the high- and low-pressure systems.
Many voices, one message
"These systems govern the everyday weather that we recognise as rainfall, snow, temperatures, cloud and sunshine.
"In that sense the data can be regarded as more fundamental to our understanding of climate than are instrumental data such as temperature and rainfall measurements."
Dr Wheeler told BBC News Online: "We've verified that the data are highly reliable. You find lots of ships sailing in convoy, and they all record the same thing.
"There's a remarkable consistency of observation. And remember, the crews' lives depended on them getting their records right.
"There are 250,000 logbooks in the UK, and we've only scratched the surface. There are far fewer in the three other countries - and virtually none in Portugal, where you might have expected many, because the Lisbon earthquake largely finished their collections."
All images copyright and courtesy of the UK National Maritime Museum.