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In 2017, the United States Environmental Protection Agency administrator, Scott Pruitt, said that human activity is not the primary contributor to global warming. However, he acknowledged that humans do play a role in climate change, but we need to find out exactly how much of one.
Despite many skeptical climate experts in the scientific community today, 97% of climate scientists agree that humans are to blame for the warming of the Earth over the last 100 years. How can they be so sure?
The Earth’s climate has naturally moved in and out of ice ages and warm periods over the last 650,000 years. Changes naturally occur in the Earth’s climate. Alterations in the Earth’s energy balance due to external factors or forcing – an environmental factor that influences the climate – cause climatic changes. The shifts in the Earth’s climate were caused by changes in ocean circulation, solar output, Earth’s orbit and title, albedo, and atmospheric chemistry.
Evidence from past environments show that the Earth’s climate has varied throughout its history. Therefore, is climate change just the Earth’s natural cycle?
Ice cores from Antarctica and Greenland provide information about climatic conditions as far back as 800,000 years ago. Scientists analyze past climatic and environmental conditions by studying these drilled ice cores. Their studies on ice cores found that rising CO2 levels are closely linked with rising temperatures.
Scientists also obtain a pretty clear picture of the Earth’s climate hundreds to thousands of years ago by studying ocean sediments, pollen remains, tree rings, and changes in the Earth’s orbit around the sun.
Today’s carbon dioxide levels are 40% higher than before the industrial revolution began. In the 18th century, atmospheric carbon dioxide concentrations were 280 parts per million (ppm). They rose to 400 ppm in 2015 and 410 ppm in 2017. Additionally, today’s atmosphere contains much more methane than at any time in the past 800,000 years. Our atmosphere has 2.5 times more methane today than before the Industrial Revolution began. Wetlands, volcanoes, wildfires, and sediments naturally emit methane. But most methane emissions today come from human activities like livestock farming, oil and gas production, and waste disposal.
Since 1880, global temperatures have risen by 1.1oC. The last few decades have witnessed the thinning and decrease of Arctic sea ice. Ice sheets in Greenland and Antarctica are rapidly retreating and decreasing in mass. Temperatures in the North and South Poles are rising faster than anywhere else on the Earth. The number of record-breaking high-temperature events in the Northern Hemisphere is increasing. Oceans are the warmest they’ve ever been in half a century. Oceans are 30% more acidic today than at the start of the Industrial Revolution. Global sea levels are rising. And here is how scientists know that all this is not a part of the Earth’s natural cycle but is due to human activity.
When analyzing the Earth’s climate, scientists observe different things at once. They have a clear understanding that the heat content in the ocean is going up. It implies that the Earth’s radiation budget must be changing – more energy must be coming in than going out.
This can happen in several ways, but each element will have a different fingerprint. Let’s look at some of the common elements.
1. The sun
For example, if the Earth were getting warmer due to an increase in the sun’s radiation, the atmosphere all the way up to the mesosphere would warm. But this isn’t happening. What scientists are observing is warming at the Earth’s surface, cooling in the stratosphere, and cooling in the mesosphere. This isn’t a signature of solar forcing. It’s a signature of greenhouse gas forcing. Moreover, we have satellites monitoring the sun. Through them, we know that the sun’s radiation hasn’t increased since 1978, even though surface temperatures continued to rise.
Additionally, when the troposphere absorbs CO2, it expands as it gets warmer. The expansion is pushing the boundary between the troposphere and stratosphere further upwards. If the sun were causing global warming, both the troposphere and stratosphere would warm.
2. Carbon isotopes
Scientists even know the difference between carbon molecules emitted naturally by plants and animals and those emitted by burning fossil fuels. Carbon molecules sourced from different materials have a different number of neutrons in their nuclei. We call these different versions of the same molecule ‘isotopes’. Carbon isotopes emitted by fossil fuels and deforestation activities are lighter than those from other natural sources. Therefore, when scientists measured carbon in the atmosphere, they observed that the level of lighter carbon molecules is increasing. They then easily linked this increase with the rise in fossil fuel emissions.
3. Ocean sediments
Even deep-sea and ocean sediments help scientists understand past changes in the Earth’s climate. Ocean sediments provide scientists with a baseline that helps them compare the past with the present. Through the study of ocean sediments, scientists have found that there have been natural variations in the Earth’s climate over the last 2,000 years. But none of these variations were very large in magnitude. Only the warming after the 1850s that scientists termed ‘remarkable’, the same time period as the dominance of the industrial revolution.
Ocean sediments and ice cores show that the Earth is warming ten times faster than before. This kind of warming did not occur even after the Earth emerged from the ice ages. The Earth is currently warming at a rate unprecedented in the last 1,300 years.
Scientists use data sets and climate models to try to reproduce the changes we are seeing today. When scientists feed the models with only natural inputs, such as solar intensity and ocean circulation, they do not obtain a result compatible with the climatic changes occurring so far. But when they feed the model with inputs like greenhouse gases, the result aligns with today’s Earth’s climate.
That can only mean one thing. Humans have caused climate change. Sure, the Earth’s climate changes naturally over a period of time. But not at the rate we’re observing today. In its history, the Earth’s climate has never changed at such a fast rate and such a great magnitude. 100% of the rises in temperatures in the last decade are due to greenhouse gas emissions.