Earlier this month, a remote-monitoring system in Hawaii recorded the first time in human history that the daily average for carbon dioxide levels in the atmosphere averaged more than 400 parts per million (ppm).
Crossing the 400-ppm line is not inherently meaningful, other than reminding us we are on a path to a place we don't want to go. But it can be an opportunity to step back and reflect on the virtues of a stable climate.
Since the birth of the industrial age, carbon dioxide emissions, mostly from fossil-fuel combustion, have steadily gathered in the atmosphere. Oscillating between 180-280 ppm for most of our history, this indicator has spiked in recent decades to its current peak of 400 ppm.
There is little chance this uphill climb will flatten anytime soon. Growing populations, increasing affluence and the global build-out of a fossil-based energy system guarantees that emissions will continue to climb rapidly.
Most scientists predict carbon levels will at least double again in a "business-as-usual" scenario, adding 4 degrees Celsius to global temperatures in the 21st century.
A recent World Bank study describes this 4-degree scenario as intolerably bleak -- rising seas, superstorms, prolonged droughts, collapsing biodiversity and dramatic impacts on people's ability to survive and prosper.
Seen in historical context, our global society has benefited enormously from a uniquely stable period in the last 12,000 years, since the last ice age. A constant and temperate climate over much of the earth's surface allowed human societies to develop agriculture and cities, and reach the level of prosperity we now enjoy.
Modern society is a direct product of the blessings of a stable climate. All of our systems, innovations and social structures are finely tuned responses to our climate. These adaptations allowed for global food production and trade, ability to control water for our purposes and to cope with weather events within a certain variable range.
With this long view, we have to ask ourselves if we are doing all we can to preserve this stability, and if we are prepared to rapidly rethink everything if this stability is sacrificed.
While we cannot be certain now of the long-term impacts of escalating emissions, protecting the climate requires us to rapidly build on and improve existing efforts. Of the many current initiatives, I would highlight three promising developments:
1. Urban revolution: Cities are now leading the climate-change response. In the wake of stalled international agreements and national climate policies, cities as diverse as Cairo, Hong Kong, Berlin and Los Angeles have partnered to take on the challenge of becoming more resilient in a climate-changed world.
The C40 Cities Initiative has brought together urban leaders from every continent to mitigate and adapt to climate change through the redesign of food, water, waste, transport, health, housing and industrial systems.
2. Valuing natural capital: Governments, banks, investors and business leaders are working together to recognize the value of natural assets like a stable climate, and include this valuation in government and business decision-making.
By putting a price on nature, it is hoped decision-makers can better recognize, protect and invest in the natural systems that support them.
3. U.S. carbon diet: In perhaps the most hopeful sign of changing times, U.S. carbon emissions are down by 13 percent in the last five years, returning to levels not seen since 1994. This trend hints at fundamental changes in how energy is produced and used. The rise of natural gas, greater efficiency, new technology and other economic factors have all helped to bend down the emissions curve. This suggests that significant changes can happen fast and at scale.
Some people wonder if the 400-ppm line indicates we're already too late. I would argue that, at a minimum, it is a wake-up call that we shouldn't ignore to build a better energy future.
Daniel Vermeer is executive director of the Center for Energy, Development, and the Global Environment (EDGE) at Duke University.