Instead of counting and measuring rings, a team of scientists, measured the levels of different forms of oxygen trapped in the wood. Oxygen in the air holds more of the lighter-weight form, or isotope, of oxygen, O-16, with smaller amounts of heavier O-18.
When water evaporated off of the ocean, some of the water rains back down onto the ocean, carrying the heavier O-18 molecules with it. The rain that reaches land carries more O-16.
Fog, on the other hand, evaporates from the ocean and blows directly onto the trees where it condenses and drips down to water the trees. Since fog doesn't condense until it reaches the trees, it carries a higher percentage of O-18.
ANALYSIS: Ancient Giant Trees Found Petrified in Thailand
The redwoods sucked up the water from both rain and fog. The ratio of the different types of oxygen corresponded to the varying amounts of fog and rain over the years. For example, a foggy yet dry summer would result in more O-18. Similarly, variations in carbon isotopes in the redwoods reflected changes in growth related to temperature and total moisture in the environment.