Warming up to climate change: How do we know humans are causing global warming?

Are humans causing global warming?

Earth has cycled through many cool and warm periods in its 4.5 billion-year history. They have been caused by several natural factors like solar energy, which varies over time and affects the climate, or ocean circulation — the movement of heat around the world by currents. For instance, around 12,000 years ago, major changes in Atlantic circulation turned the Northern Hemisphere into a “frigid state”, according to a report by The New York Times.

Volcanic activity can also significantly alter the planet’s climate. Eruptions lead to the release of gases and dust high into the atmosphere, which reflects back the sunrays and results in short-term cooling of Earth. The opposite can happen too. A dramatic period of global warming 56 million years ago was triggered due to a volcanic eruption that released large amounts of carbon dioxide (CO2) — global temperatures soared by up to five degree Celsius.

But are these natural factors responsible for current global warming? Although they are still at play, their influence is too little or they occur too slowly to explain the rapid warming, especially in recent decades, according to NASA.

The main culprit behind the soaring global temperature is rising greenhouse gas concentrations in the atmosphere. As explained in the first instalment of this series, greenhouse gases like CO2, methane (CH4), and water vapour trap the Sun’s energy in the Earth’s system before it escapes to space, causing warming. Since the industrial revolution, human activities like burning fossil fuels such as coal, oil, and gases have released more and more greenhouse gases.

In this figure, the black line shows the observed annual average global surface temperature for 1880–2017 as a difference from the average value for 1880–1910. The other lines show the contributions from individual natural and human factors, all natural factors, all human factors, and the combined effects of both natural and human drivers. Source: US Global Change Research Program.

Take the example of CO2. Its concentration in the Earth’s atmosphere has spiked dramatically over the last 150 years, from a pre-industrial era concentration of approximately 280 parts per million (ppm) to more than 410 ppm currently, according to a report by the Union of Concerned Scientists.

Moreover, measurements from ancient ice cores show that CO2 is now at its highest levels in over 800,000 years. As a result, the planet has experienced rapid warming, particularly since the 1950s.

Carbon dioxide levels. Credit: NASA

In 2014, the UN’s Intergovernmental Panel on Climate Change came out with a report that said: “Human influence on the climate system is clear and growing, with impacts observed across all continents and oceans. Many of the observed changes since the 1950s are unprecedented over decades to millennia. The IPCC is now 95 percent certain that humans are the main cause of current global warming.”

Over the years, more and more studies have confirmed the role of human activities in worsening global warming. A 2014 study, ‘A probabilistic analysis of human influence on recent record global mean temperature changes’, found that the odds of current warming occurring without anthropogenic (caused by humans) greenhouse gas emissions are less than 1 in 100,000.

To read the first part of the series that addressed the question if the Earth is becoming warmer, click here.