It only took one person to change a light bulb over the famous black door at Number 10 Downing Street, but the new bulb could save even more energy.
Downing Street said most of its bulbs are already energy saving
The new low-energy bulb is all part of an environmentally-friendly campaign going on inside Number 10 and Chequers, the prime minister's country residence.
Mr Blair has agreed to take part by turning down his home thermostat and reducing carbon emissions on flights.
The moves are part of National Science Week which runs until 19 March.
Downing Street is participating in the Click for the Climate element of the week and has decided that from 1 April all flights taken by ministers and civil servants will be carbon neutral.
Energy-saving is not a new idea at Number 10, according to a spokesman.
"Most of our light bulbs are already energy saving - the change started gradually last year.
"The bulb in the lantern was still working (when it was removed) but I am sure it will be put to good use elsewhere."
Mr Blair said: "I am pleased to support Click for the Climate and will be turning down the thermostat by one degree to reduce my personal energy usage.
"This continues the precedent we set by making the G8 Gleneagles conference last year entirely carbon neutral.
"In addition, Downing Street already sources 75% of our electricity from green suppliers, and as many light bulbs as possible have been changed over to energy-efficient ones."
'Pennies make pounds'
Click for the Climate is an online pledge allowing people to see how much they can reduce their carbon dioxide emissions and how much has been pledged across the country.
It was launched by the British Association for the Advancement of Science and the Economic and Social Research Council (ESRC) in association with CRed (the Carbon Reduction programme at the University of East Anglia).
Others joining the campaign include former Olympic athlete Colin Jackson, who will be taking the train instead of flying and TV scientist Lord Winston is going without his car.
Members of the public are being invited to take part and make a small change in their lives during the week and make a big difference to climate change.
British science association chief executive Roland Jackson, said: "If they all just replaced one ordinary light bulb in their home with an energy-saving bulb, then National Science Week would have cut down carbon dioxide emissions by over 240 tonnes.
"People say they can't make a difference - this shows that they clearly can."
ESRC chair Frances Cairncross, who is also president of the British science association, said: "People don't always realise that even the smallest of actions can make a difference.
"Just as 'pennies make pounds', so switching off your TV at the button instead of leaving it on stand-by, or walking to school instead of getting a lift, could help save thousands of tonnes of carbon dioxide if everyone took similar steps."