2001: Wind, solar and geothermal energy produce 0.5% of the nation's electricity. The Department of Energy forecasts that by 2020 this will double — to roughly 1%.

2002: California, the nation's most populous state, requires 20% of its electricity to come from renewable energy by 2017. California's move nearly triples the amount of renewable electricity to which states have committed by 2020.

2003: State renewable energy targets would result in 2.4% of the nation's electricity coming from renewables by 2020.

2004: Five more states adopt renewable electricity standards, with Colorado becoming the first to do so by popular vote.

2005: Texas moves to invest billions of dollars in electric transmission capacity, unlocking a massive expansion of wind energy.

2006: California adopts Million Solar Roofs Initiative, dedicating $3 billion to fueling expansion of solar energy. Meanwhile, Washington and Maine adopt new renewable electricity targets while Arizona and New Jersey increase theirs, leading to a 19% jump in states' renewable energy commitments.

2007: Five more states adopt renewable electricity standards, increasing states' 2020 renewable electricity commitments by another 25%. Wind, solar and geothermal power produce 1.2% of nation's electricity, twice as much as in 2001.

2008: Missouri voters adopt 15% renewable energy target by 2021, joining Ohio and Michigan in adopting new renewable targets.

2009: American Recovery and Reinvestment Act provides $25 billion in renewable energy grants, sparking clean energy during the recession.

2010: Colorado increases its renewable goal for second time, to 30% by 2020. Six other states increase or amend their renewable targets.

2011: California increases its renewable electricity goal to 33% by 2020 amid a wind and solar energy boom in the state.

2012: Wind power surpasses gas to become the number one source of new electricity generation capacity in America.

2013: Wind, solar and geothermal exceed 4% of U.S. electricity supply, more than eight times the level in 2001.

2014: Burlington, Vermont, becomes first U.S. city to be powered by 100% renewable electricity.

2015: Hawaii becomes first state to adopt a statewide 100% renewable electricity standard.

2016: Wind, solar and geothermal exceed 7% of U.S. electricity supply, more than doubling over the previous five years.

2017: Wind and solar energy provide 10% of America's electricity in March and April, cracking that threshold for the first time ever.

2018: California sets a standard of 100% clean electricity by 2045. And the nation surpasses its renewable energy targets for 2020, two years early.

2019: Five more states (as well as Washington, D.C.) adopt 100% clean electricity requirements.

2020: Wind, solar and geothermal power exceed 10% of annual U.S. electricity supply (based on 2019 data).

That's enough to power all the homes in 423 cities the size of Des Moines. But there's still a long way to go. Here's what could happen next...

2030: Thanks to state clean energy targets and improvements in renewable energy, America is on track to get 23% of our electricity from wind, solar and geothermal power by 2030, according to the U.S. Energy Information Administration.

2040: We are on track to get 26% of our electricity from the wind, sun and earth by 2040.

2050: And we will get nearly a third of our electricity from solar, wind and geothermal power by 2050.

But a third is not enough. We have the resources and the know-how to set a goal of achieving 100% renewable energy. And with dirty energy sources putting our health and our climate at risk, we need to start now.