2015 featured some of the most significant climate milestones in human history. From record-high temperatures, to atmospheric concentrations of carbon dioxide not seen in a million years or more, to a landmark international agreement to limit global warming, no other year has seen such a stark contrast of climate indicators. Each milestone provides perspective on how far we’ve come, where we are today and what the future might hold.

Warmest Year on Record

Driven by a combination of El Niño and human-induced warming, 2015 will be Earth’s hottest year ever recorded. Many other temperature records tumbled:

  • July was the warmest month of any month ever recorded.
  • February, March, May, June, July, August, September, October and November – likely December as well – all reached record highs for each respective month.
  • October and November were the first months on record to exceed their respective monthly long-term averages by more than 1 degree C (1.8 degrees F).
  • 2015 capped off the warmest five-year period on record, following the last three decades that were successively warmer than any preceding decade on record.

More than 1 Degree C Above Pre-industrial Temperatures

Under the Paris Agreement, countries have agreed to limit warming well below 2 degrees C (3.6 degrees F), and strive for 1.5 degrees C (2.7 degrees F), above pre-industrial levels to avoid some of the worst impacts of climate change. In addition to unparalleled global warmth, 2015 marked the first time in recorded history that the world’s annual temperature climbed past the halfway point to the 2 degrees C threshold. We are even closer to this threshold given that the Earth’s ability to store energy means warming will continue. For example, the ocean has absorbed more than 90 percent of excess heat from global warming since 1970, so even if all emissions stopped today we would continue to warm around 0.6 degrees C (1.08 degree F).

Step back, climatologically, and consider:

  • We have already burned more than two thirds of our “carbon budget,” or the amount of carbon dioxide the world can emit while still having a likely chance of limiting warming to 2 degrees C. If emissions continue unabated we will spend the rest of the budget in less than two decades.
  • It’s been nearly 40 years since annual global temperatures were below average.
  • Fourteen of the 15 warmest years on record have occurred since 2000.

With the impacts of climate change already being felt around the world today, exceeding this climate milestone underscores the urgent need for substantial reductions in global greenhouse gas emissions.

Carbon Dioxide Tops 400 ppm

The buildup of greenhouse gas emissions leads, much like a thickening blanket, to warming of our climate. Carbon dioxide is the primary driver of human-made warming and measurements of the global atmospheric concentration of carbon dioxide provides insight into how much we are changing our climate.

For the first time in recorded history, in March, the global concentration of carbon dioxide in the atmosphere reached 400 parts per million (ppm). While 400 ppm had been reached at Hawaii’s Mauna Loa Observatory in 2013, this year was the first time it was over that level as a global monthly average; Southern Hemisphere measurements have lagged Northern Hemisphere measurements due to fewer sources of human carbon dioxide emissions. Consider:

  • The global carbon dioxide concentration is 24 percent higher than when record-keeping began in 1958 and 120 ppm higher than before the Industrial Revolution, with half of that rise occurring since 1980.
  • Each year, the global carbon dioxide concentration rises by more than 2 ppm. This rate has accelerated in recent years and is more than double the rate of the 1960s.
  • Advocates have adopted varying targets for the global carbon dioxide concentration, including many calling for limiting it to 350 ppm – which we have long ago surpassed – and others 450 ppm – which we are quickly advancing towards.

Heads of State Gathered for Climate Change Action

World leaders gathered in Paris to forge an agreement on climate change. Nearly 150 leaders, including heads of state of the countries that emit the most, came together on November 30 in Paris in the largest gathering of heads of state ever to attend a UN event in a single day. Leaders also took part in other high-level events that launched several initiatives including:

  • An International Solar Energy Alliance to mobilize over 1 trillion dollars by 2030 for deployment of affordable solar energy.
  • An initiative of twenty major economies and the private sector called Mission Innovation to double current investments in global clean energy innovation.
  • A call on companies and countries to put a price on carbon to drive investment into low-carbon behavior.

187 Countries Make Climate Commitments

In advance of the Paris meeting, countries around the world put forward post-2020 climate change commitments. One hundred sixty INDCs representing 187 countries including the 28 Member States of the European Union have been submitted. Only 73 pledges were submitted to the UN for the pre-2020 period, so this represents a significant increase.

Some of 2015’s biggest milestones are ones we hoped we’d never see. Yet progress made in bringing climate change to the top of the international agenda offer signs of a positive shift in the future. The Paris Agreement is the most prominent positive sign for 2015. How it is implemented will determine whether we will continue to see temperature records smashed or be able to ring in future new years by seeing how far we have come.