“The Earth has never warmed this fast. We need to see how much we can change the world in the next 20 years.”

There is "no reliable documentary evidence for the quotation", according to an article in The New York Times. Brian Morton, "Falser Words Were Never Spoken" http://www.nytimes.com/2011/08/30/opinion/falser-words-were-never-spoken.html?_r=0, New York Times, 2011-08-29. It is not found as a direct Gandhi quotation in the 98-volume authorized Collected Works of Mahatma Gandhi. Misquotes that Bapu is forced to wear http://articles.timesofindia.indiatimes.com/2011-10-03/ahmedabad/30238203_1_bapu-tushar-gandhi-gandhiji
The earliest evidence for quotes of this type comes from the "Love Project", an initiative begun at 1970 at a high school in Brooklyn, New York by teacher Arleen Lorrance. According to the project's website http://www.consciousnesswork.com/love.htm, "Be the change you want to see happen, instead of trying to change anyone else" was one the principles of the Project "received" by Lorrance in 1970 -- but contemporaneous evidence for this has not been found.
A 1972 newspaper article states: "Instead of advocating change in people and things, ... Love Project encourages people to actually be change itself". [San Antonio Express, 1972-09-28, 76, 'Love Project' Marks End of Quest, Ron, Fulkerson, https://www.newspapers.com/clip/12194386/love_project_1972/]
In 1974, Lorrance wrote, in a report on the Project: "One way to start a preventative program is to be the change you want to see happen." ( "The Love Project" https://books.google.com/books?id=NcTimfiMzYUC&pg=PA85&lpg=PA85&dq=arleen+lorrance+love+project+1972&source=bl&ots=X5fggiqrCZ&sig=JoOzC2X1QU1eePkOBoy-60rJ1RE&hl=en&sa=X&ved=0ahUKEwiAwIv-_a_MAhUBW2MKHYBQDFIQ6AEIQDAH#v=onepage&q=%22be%20the%20change%22&f=false, in Kellough (ed.), Developing Priorities and a Style, MSS, 1974).
In 1976, a newspaper report listed "'Be the change you want to see happen, instead of trying to change anyone else" as one of the principles of the Love Project. 'A Ministry Called "The Love Project", St Louis Post Dispatch, 1976-11-15, p. 36
In 1987, a similar quote was attributed to Gandhi in a New Mexico newspaper: "You must be the change you wish to see in the world". Hollis Engley, "A Long List of Varied Accomplishments" https://www.newspapers.com/clip/5211946/a_long_list_of_varied_accomplishments/, The New Mexican, Santa Fe, NM, 1987-01-11, p. D-1
In 1991,"We must be the change we wish to see in the world" is attributed to Gandhi in Stella Cornelius, "Partners in Conflict Resolution", from Barnaby (ed.), Building a More Democratic United Nations (1991) Google Book link https://books.google.com/books?id=rcYYoLsFNmAC&pg=PA70&dq=%22be+the+change%22+%22wish+to+see%22&hl=en&sa=X&ved=0CB0Q6AEwAGoVChMI4ufD3tSLyQIVFFJjCh1muQX6#v=onepage&q=%22be%20the%20change%22%20%22wish%20to%20see%22&f=false
Arun Gandhi, grandson of Mahatma Gandhi, has attributed the quote to his famous grandfather since at least 2000 https://web.archive.org/web/20000823060048/http://www.jca.apc.org/g21/panelists.htm. See also "Arun Gandhi Shares the Mahatma's Message" by Michel W. Potts, in India - West [San Leandro, California] Vol. XXVII, No. 13 (1 February 2002) p. A34, and "Be the change you wish to see: An interview with Arun Gandhi" by Carmella B'Hahn, Reclaiming Children and Youth [Bloomington] Vol. 10, No. 1 (Spring 2001) p. 6.< It is not clear whether Arun claims to have directly witnessed his grandfather saying it, or whether he heard of it second-hand.
Misattributed

Michael Franti Interview http://www.youtube.com/watch?v=5s75CPceiCw&feature=related

"One Half of a Manifesto," The New Humanists: Science at the Edge (2003)

Source: Billions and Billions: Thoughts on Life and Death at the Brink of the Millenium (1997), Chapter 11, "Ambush: The Warming of the World"
Context: If we keep on with business as usual, the Earth will be warmed more every year; drought and floods will be endemic; many more cities, provinces, and whole nations will be submerged beneath the waves — unless heroic worldwide engineering countermeasures are taken. In the longer run, still more dire consequences may follow, including the collapse of the West Antarctic ice sheet, and the inundation of almost all the coastal cities on the planet.

Source: Reel to Real: Race, Sex, and Class at the Movies

Third Session of Parliament (June 30, 2007)

As quoted in "The Danger of Cosmic Genius" https://www.theatlantic.com/magazine/archive/2010/12/the-danger-of-cosmic-genius/308306/ by Kenneth Brower, The Atlantic (December 2010)
Context: There’s very good news from the asteroids. It appears that a large fraction of them, including the big ones, are actually very rich in H2O. Nobody imagined that. They thought they were just big rocks … It’s easier to get to an asteroid than to Mars, because the gravity is lower and landing is easier. Certainly the asteroids are much more practical, right now. If we start space colonies in, say, the next 20 years, I would put my money on the asteroids.

How Many People Can Live on Planet Earth? (BBC Horizon, 2009)