How do we study climate change?
PIRE CREATE Project & Climate change
We are using PIRE funding to merge data from the largest tree-ring and cave sediment archives in North and South America and compare them with simulations of past, present and future climate change based on the latest generation of global climate models. Newly developed reconstructions of historical extreme weather events (droughts, flooding, El Niño, etc.) over the Americas during the past millennium will allow us to analyze societal responses and better predict future events based on past model-archive comparisons.
“Climate change can be studied in many different ways. How scientists do this usually depends on the kind of question they want to answer”.
Prof. Mathias Vuille, PIRE Director
How has climate change varied in the recent past and how is it changing today?
To answer this question, climate scientists use observations gathered from around the globe. On land these observations stem from thousands of weather stations deployed around the world, measuring how air temperature, precipitation and other climate variables change over time. Over the oceans observations are made by ships, moored and floating buoys and by tidal gauges along the coastlines. Across the globe radiosondes are launched twice a day to obtain vertical profiles of temperature, wind and humidity of our atmosphere. Since the 1970’s our planet is also being observed from space, with a variety of different sensors mounted on satellites that either monitor a specific region of our planet (geostationary) or orbit around it (polar orbiting). All these data extend back between 50 and 150 years and give us a clear picture of a planet that is warming at an accelerating pace, leading to rapidly shrinking Arctic sea ice and glaciers, thawing permafrost, a more humid atmosphere , and a rising global sea level.
How will climate change going forward?
Since observations of the future do not exist, we have to use climate models to obtain a glimpse into the future. Climate models are mathematical tools that can simulate the climate of our planet in a simplified way using physical equations and semi-empirical parameterizations. They are able to represent the large-scale atmospheric and oceanic circulation, and their interactions with other Earth components, such as ice sheets, land surface and the biosphere. They are useful to simulate how our climate will change if we increase greenhouse gas emissions. Since we do not know how much of these gases we will emit going forward, climate scientists use different emission scenarios to simulate a broad range of possible future outcomes, called ‘climate projections’. Before doing this, climate models are tested to see if they can adequately simulate the climate conditions of the recent past, for which we have good observations, in so-called ‘model validation’ and ‘model intercomparison’ projects.
NASA's Goddard Space Flight Center. Visualization of how carbon dioxide moves through Earth’s atmosphere.
How do we know that humans are to blame for the observed climate change?
Climate models are also used to better understand why our climate is changing. Climate scientists can run the same simulation several times, but add or remove factors from the simulation that are considered important for our climate. By doing this, climate scientists have shown that no climate model can adequately simulate how climate over the past 150 years has changed without accounting for man-made greenhouse gas emissions. Natural forcing factors, such as the sun or volcanoes alone cannot explain the climate changes that have been observed over the past 150 years. This is a powerful argument for the dominant human role in the climate change that we currently observe.
How do we know that the currently observed climate change is something unusual?
While we have observations of how climate has changed in the past (see above), these observations only go back ~150 years. Throughout this entire time period humans were already emitting greenhouse gases. Therefore, almost all of our observations are tainted by these emissions and thus we do not have good observations of natural climate variability undisturbed from human influence. Furthermore, 150 years is not a long enough period to really understand how much and how fast climate can change naturally. Climate scientists therefore rely on natural archives or proxies to study how climate changed several hundreds or thousands of years ago. Natural archives are recorders of past changes in our environment. Tree rings, ice cores, corals, stalagmites, lake and marine sediments or pollen are all examples of natural archives. In order to understand how they record past changes in climate, these archives first need to be compared or calibrated with recent observations. In addition, some archives can provide us with information on how factors that may contribute to climate change (so-called ‘forcings’) have varied in the past. Tiny air bubbles, trapped in polar ice cores from Antarctica, for example, hold continuous records of the greenhouse gas concentration in our atmosphere over the past 800,000 years. They show that since the start of the industrial revolution, greenhouse gas concentrations in our atmosphere have soared to levels that are now 40% higher than they have ever been in the past 800,000 years.