Outreach & Policy

Current climate change is well observed, in that we have very accurate measurements from around the globe of how key climatic quantities such as temperature, rainfall, and sea level are changing, and how they have changed over the last 150 years.  These show, for example, that since 1850, atmospheric CO₂ has increased by 45% and surface temperature has increased by ~1^oC.  Since 1900, sea level has increased by ~20cm.  But how unusual are these recent changes in the context of longer timescales?  The geological record provides an ideal archive for answering this question. 

For CO₂, ice cores from Antarctica that contain trapped bubbles of ancient atmosphere provide the most accurate records of past change.  They indicate that the current concentrations of atmospheric CO₂ are higher than at any time in at least the last 800,000 years (the limit of the depth of current ice cores).  Further back in time, ancient fossils that record the CO₂ concentration in the ocean can be used to reconstruct past atmospheric CO₂.  Although more uncertain than ice cores, these records indicate that current CO₂ concentrations, which have been rising since the industrial period, are currently higher than at any time in the last 2 million years.  Some scenarios of future CO₂, in which little or nothing is done to reduce carbon emissions, see the CO₂ concentration by the end of this century approaching that of the Eocene, 50 million years ago, when there was little or no ice anywhere on the planet.

For temperature, multiple methods can be used to reconstruct past temperatures.  For the last 2000 years, the most commonly used methods are those of tree-ring and coral analysis.  Trees and corals respond to temperature in terms of their growth characteristics (e.g. the width of tree-rings), and can therefore be used to reconstruct past climate at specific locations.  When many of these records are combined, and account taken of regions with relatively little data, PAST global average climate can be reconstructed.  These analyses indicate that the current temperature is warmER than at any point in at least the last 2000 years.  Temperatures now are significantly warmer than during the “Medieval Warm Period”, and are still increasing at a rapid rate (see Figure 1).  If humanity chooses a high-carbon future, then in just a few hundred years we will send our planet into a state similar to that of the Pliocene, 3 million years ago.

The Paris agreement looks at limiting warming to under two degrees Celsius. This target refers to average temperatures around the Earth. We know from previous periods of climate change that some regions are likely to experience far greater warming than others. Likewise, some areas may be more prone to flooding or drought than others.

The high latitudes are subject to an effect known as polar amplification, which causes temperatures there to rise faster than in the mid latitudes. Polar amplification is caused in part by melting glaciers and ice sheets, which allow more sunlight to be absorbed by the bare ground. Rapid temperature rise can cause permafrost to melt, damaging roads and buildings and potentially releasing stored greenhouse gases.

Rising ocean and air temperatures can alter the strength and timing of the West African and Indian Monsoons, which are crucial for the socio-economic stability and food and water security of billions of people. Warmer temperatures also causes an intensification of the climatic phenomenon known as the El Niño–Southern Oscillation (ENSO), which influences flooding, droughts, food supplies and wildfires across the world.

Predicting our future climate is crucial to allow us to plan for the future, for example to decide how large flood defences need to be, or which cities may become too hot to live in. These predictions of the future climate are made by climate models. Climate models are computer programs that include our very best understanding of how physics, chemistry, and biology interact on our planet to produce weather and climate patterns. Climate models need to be tested, and this testing is typically carried out by setting up the model to predict known changes over the last 150 years or so. Over this period we have direct observations from weather stations (and more recently satellites) of how climate has changed.

However, our future climate is likely to be very different to the last 150 years, with CO2 concentrations and temperatures increasing to be higher than those we have previously experienced. This is where geology comes in! The geological record, for example pollen grains preserved in ancient lakes, or microscopic fossils preserved in the sediments at the bottom of the ocean, provide a window into more ancient climates, and in particular climates that may resemble those of our future. A time period that has been used for this purpose is the Eocene (about 50 million years ago), which had CO2 concentrations similar to those we might expect by the end of this century under intensive fossil-fuel scenarios. The Eocene was exceptionally warm, with little or no ice at either pole, and scorching temperatures in the tropics. By setting up our climate models to represent this time period, and comparing the results with information from the geological record, we can test our Climate Models under high-CO2 conditions. This shows that our models do a good job of predicting the global average temperature of this time period, and also the amount of warming in the poles compared with in the tropics. As such, we can gain confidence in the models’ predictions of the future, and can therefore make informed plans about future defences against climate change, and plan for a low-carbon future.

On the order of months to years, the Earth’s climate is governed by a number of phenomena such as the El Niño-Southern Oscillation, the North Atlantic Oscillation and the Southern Annular Mode. These phenomena are characterised by fluctuations in the locations of atmospheric pressure systems, which in turn alters wind speed, precipitation, and local temperatures.

Climate can also be affected by so-called external events, such as large volcanic eruptions and changes in solar activity. Large volcanic eruptions release particles into the atmosphere that can block sunlight and lead to short-lived cooling. For example, the eruption of Tambora volcano in 1815 released ash high into the atmosphere, and led to what was known as the ‘year without a summer’ in 1816. Periods of low solar activity can alter the jet stream, causing colder winters in northern Europe and milder winters in southern Europe. Such changes can affect how we grow food and how much energy we might need to heat our homes.

Scientists have used corals, tree rings, lake and ocean sediments and ice cores to document both types of variability, particularly over the last 2,000 years. These records help them to understand how warming temperatures might enhance or reduce naturally occurring climate variability.