Humans and Global Warming

By Matthew Stepp and Amanda Kibbe, Center for Clean Energy Innovation

In 2012, Jesse Jenkins and Matthew Stepp took stock of the global climate policy challenge in an online series titled The Future of Global Climate Policy. Since then the Intergovernmental Panel on Climate Change (IPCC) completed its Fifth Assessment and many countries are taking stock of their existing—and some argue, failed—climate policies. Looking to the future, the latest round of international climate negotiations is set to close in Paris at the end of 2015, potentially offering the end of one era of global climate policymaking and the start of something new. With an eye on the long-term impacts of the 2015 negotiations, Amanda Kibbe and Matthew Stepp take an updated look in a five-part series on the state of the climate challenge.

Global climate change is often difficult to assess. The weather changes daily and global climate changes on long timescales. Regions and countries are impacted differently. It is challenging to express complex, long-term climate trends in comparison to daily and seasonal changes in the weather that the general public experiences. But with each passing year, the science, observations, and modeling of global climate change become even clearer: humans continue to influence global climate change by pumping greenhouse gases into the atmosphere, which increases Earth’s average temperature.

The Intergovernmental Panel on Climate Change (IPCC)—the United Nations sanctioned scientific body tasked with synthesizing the mountains of scientific literature on climate change—completed its Fifth Assessment in 2014. It’s overwhelming scientific consensus is more confident than ever that the climate is changing, going so far as to describe the warming of the Earth’s climate as “unequivocal” and “unprecedented,” and that human activity is the cause. In an aggressive rhetorical flourish, the IPCC states that, “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”

To be clear, the IPCC’s confidence isn’t much of a profound statement of new scientific theory. Rather, it is a renewal of scientific fact long-argued for much of the last decade that is now being played out in Earth observation data and weather events, and often to a much more dangerous degree than originally predicted. Of course, capturing all of the factors that shape the world’s climate is a monumental task; we will summarize some of the main indicators captured in the IPCC report:

Increase in greenhouse gas (GHG) concentrations:The concentration of greenhouse gases (GHG) in the atmosphere continues to increase.  In 2011, the concentration of carbon dioxide (CO2)—the most significant GHG emitted into the atmosphere—reached 390.5 ppm or 40 percent higher than concentrations measured prior to the industrial revolution (280 pm). In April 2014, global CO2 concentrations hit 402 ppm, the highest levels in over 800,000 years. There is near universal consensus in the scientific community that this increase in GHG and CO2 concentrations, due to human activity, has triggered a series of dynamic changes in the Earth’s climate system.

Increasing Ocean and Air Temperatures: As a result of increasing GHG concentrations, global mean surface temperature (both land and oceans) has increased by 0.85°C since the late 19th century. More specifically, the last three decades have been the warmest period of the past 1400 years. The number of warm days and nights has increased on a global scale, resulting in fewer cold days and nights. And because of better data collection since the Fourth Assessment, the IPCC is also certain that global average sea surface temperatures (SST) have increased as well.

Changes in Extreme Weather:  In the United States, the impacts of extreme weather events like Superstorm Sandy and Hurricane Katrina are beginning to leave a lasting impression of climate change. And while economic and population growth also play a significant role in the overall impact of extreme weather events, the IPCC states that heat waves, heavy precipitation events, tropical cyclones, and drought have increased in many regions across the world. With only a few exceptions, wet regions are becoming wetter, and dry regions are becoming drier, exacerbating impacts of both floods and droughts around the world.

Loss in Ice Sheets, Glaciers, Sea Ice Extent, and Snow Cover: With the rise in air and ocean temperatures, it’s no surprise that the planet’s ice and snow has begun to melt. In the past 20 years, ice sheets in both Greenland and Antarctica have been losing mass. The rate of ice loss in the most recent decade was eight-times faster in Greenland, and nearly five-times faster in Antarctica, compared to the prior decade. Similarly, glaciers, Arctic sea ice, and snow cover in the Northern Hemisphere have also decreased substantially.

Rise in Sea Level: Accounting for ocean currents, wind, and geography, global sea levels have risen, largely due to the loss of glacier mass, sea ice, and ice sheets, as well as ocean expansion due to warmer temperatures. According to the IPCC, sea levels have risen by 1.7 mm/year since 1901, a faster rate since the mid-19th century compared to the previous two millennia. In the last 20 years, the rate of sea level rise has nearly doubled.

In the previous series on the Future of Global Climate Change, the consensus was, “dangerous warming is coming, if not already here today.” It might sound like a broken record, particularly within the climate policy debate in Washington, but the main takeaway from the IPCC’s 2014 consensus on climate science is: climate change is real, it’s already happening, and it’s accelerating. The question remains how badly the climate will change, not whether it will change at all. In Part 2 of the series, we’ll take a look at the IPCC’s projections of potential impacts of climate change if the world doesn’t act fast.