Many native British species are struggling to cope with the "stop-start spring", wildlife experts say.
The Springwatch project hopes to achieve 100,000 wildlife sightings
A survey involving 65,000 wildlife sightings suggests that frogs and bumblebees are among the hardest hit.
"Climate change is not something that is happening a million miles away - it is going on in our own back gardens," said nature presenter Bill Oddie.
The findings come from Springwatch - a project being run by the Woodland Trust and the BBC between January and June.
Over 30,000 volunteers have noted their first sightings of six key species, passing the data on to researchers studying how nature is responding to global warming.
Since January the public has been invited to record first sightings of bumblebees, frog spawn, seven spot ladybirds, peacock butterfly, hawthorn flowering and swifts arriving.
Early reports suggest large fluctuations in temperature have resulted in widespread sightings of frozen frogspawn.
"If it has died it means that frogs that produced that spawn aren't going to have another chance to breed that year," said BBC environment correspondent Sarah Mukherjee.
There are also concerns that some bumblebees may have been killed off, despite new reports of active nests throughout the winter.
Bumblebees have been recorded since mid-December, while frog spawn was seen in Cornwall on 1 November, several days earlier than other recent warm years and about two weeks earlier than 2001.
The peacock butterfly, not normally seen until March, has been spotted widely, with a first sighting on 28 December and well over 2,000 recordings.
The Royal Society for the Protection of Birds (RSPB) said the recent cold snap had slowed down nesting.
"When I was a lad we had 'proper' winters and spring started in April. Now that seems a thing of the past," said Bill Oddie.
"When compared to records from previous years, these results show that some of our insects and plants are appearing several weeks earlier."
He said the change may have "knock-on effects for the birds and other wildlife that depend on them for food".
Nick Collinson of the Woodland Trust said the aim was to get more than 100,000 recorded sightings by the time the Springwatch project is completed in June.
He said the response to Springwatch had been "amazing".
"We're getting a fantastic picture of spring emerging across the UK."
The initial results are broadcast on Springwatch at 1900 GMT on Friday on BBC Two.