A Europe-wide study has provided "conclusive proof" that the seasons are changing, with spring arriving earlier each year, researchers say.
Feeling the heat: Plants' behaviour is affected by the climate
Scientists from 17 nations examined 125,000 studies involving 561 species.
Spring was beginning on average six to eight days earlier than it did 30 years ago, the researchers said.
In regions such as Spain, which saw the greatest increases in temperatures, the season began up to two weeks earlier.
The findings were based on what was described as the world's largest study of changes in recurring natural events, such as when plants flowered.
The team of researchers also found that the onset of autumn has been delayed by an average of three days over the same period.
Feeling the heat
The study, published in the journal Global Change Biology, shows changes to the continent's climate were shifting the timing of the seasons, the scientists said.
One of the paper's lead authors, Tim Sparks from the UK's Centre for Ecology and Hydrology (CEH), said the findings did not go as far as pointing the finger of blame at human-induced climate change.
"We can't tell that from our study but experts have already shown that there is a discernable human influence on the current climate warming."
But Dr Sparks said it did show that there was a direct link between rising temperatures and changes to plant and animal behaviour.
"We need to look at change over very large areas and we need to examine as many species groups as possible because there has been some mild criticism that people have cherry-picked the results they presented.
"We have gone for the most complete coverage possible that we could in Europe to try to see if there was still this effect," he said. "It is very conclusive that there is."
The team examined 125,000 observational series of 542 plants and 19 animal species in 21 European countries from 1971 to 2000.
The results showed that 78% of all leafing, flowering and fruiting records were happening earlier in the year, while only 3% were significantly delayed.
Dr Sparks said horse chestnut trees, which grow all over the continent, were particularly good indicators.
"It is a good example because it is easy to identify, and it has distinctive phases of leafing, flowering and producing conkers."
He hoped the findings would now focus attention on the potential consequences of changes to the behaviour of plants and animals.
"If you have species that are dependent on each other changing at different rates, that could just break down the food web.
"For example, caterpillars feed on oak trees, and birds feed on the caterpillars. Unless these species remain synchronised, there could be problems for any one or more of those elements of the food web."