The Holocene: Scientists meet to discuss reconstructing the climate of the past
Knowing how and why earth’s climate has changed in the past is key to understanding the impact of human activity now and in the future. Last week, scientists gathered at the Geological Society in London to discuss earth’s climate over the last 11,700 years – known as the holocene. We went along and listened to the discussions. Here’s a quick look at what reconstructing earth’s history is all about.
Climate proxies
In new research published in Science Express, a team of scientists led by Lonnie Thompson of Ohio State University studied an ice core drilled from the Quelccaya Ice Cap in Peru, the world’s largest tropical ice sheet. Unlike previous ice cores from the same area, each year of the core’s 1,800 year lifetime is recorded in clearly defined layers. By examining the layers, the researchers can learn about the climate conditions in the region over past two millennia.
This is one example of what scientists call a climate proxy.Thermometers have only been measuring temperature directly since about the late 17th century. So instead scientists use natural recorders of temperature change, which can include seafloor sediments, corals, fossilised pollen on land, ice cores and tree rings.
The conference at the Geological Society centred around how proxies can be used to reconstruct various aspects of earth’s climate over the holocene.
One talking point was a recent paper in Science by Shaun Marcott from the University of Oregon that used a range of climate proxies to reconstruct global temperature over the holocene.
Image - Marcott Et Al ., (2013) (note)
Source: Marcott et al., (2013)
The authors concluded that global temperature rose by about 0.6 degrees Celsius over the first half of the holocene, peaking about 7,000 years ago, and then declined until about 200 years ago. We reported on the paper here.
While it’s known that natural changes in the sun’s position relative to earth drive temperature changes over timescales of thousands of years, many of the talks at the conference described how other parts of the climate system, such as atmospheric and oceanic circulations, influenced the size and speed of the changes.
Short, sharp changes
While the holocene is generally considered quite a stable period in earth’s history, there are some interesting events permeating the temperature record. One example is a period of global cooling between about 1400 and 1850, known as the Little Ice Age (LIA).
One of the main questions scientists are seeking to resolve using climate proxies is how widespread such events were, how long they lasted and how they compare in magnitude to today’s climate.
A talk by Paula Moffa Sanchez from the University of Cardiff on Thursday highlighted the interlinking factors that led to the onset of the LIA, which saw temperatures drop in some parts of the northern hemisphere by as much as two degrees Celsius.
Moffa Sanchez explained that a major atmospheric circulation known as the North Atlantic Oscillation (NAO) was in a negative phase at the onset of the LIA, which amplified the cooling effect of a reduction in solar irradiance and volcanic activity. This, in turn, affected the distribution of sea ice in the Arctic and disrupted a major ocean circulation that distributes heat across the globe, known as the Atlantic Meridional Overturning Circulation ( AMOC).
As with other events during the holocene, it seems the NAO played an important role in the onset of the LIA. But the feedbacks between different processes are what scientists are trying to get to the bottom of. Moffa Sanchez’s research is currently being reviewed for publication by Nature Geoscience.
Challenges
Studying proxies isn’t all about global temperature. There are changes in sea level rise, ocean circulation, vegetation and ice cover to think about too – and other proxies can help.
A keynote talk by Ian Hall from Cardiff University opened the conference by describing how the grain size of silt from sediment cores taken from the seafloor can be used to infer how deep ocean currents that form part of AMOC have changed in strength over time.
But while climate proxy records can tell us a lot, intact records that extend far back in time are few and far between. So determining how the climate changed over the holocene can still be difficult.
For example, Antony Long from Durham University discussed the problems of reconstructing past global sea level change. With examples from recent literature, Long highlighted how proxies from different areas can indicate different things about how sea level changed. Determining how and why different proxies vary – does it mean real differences or differences in techniques? – is still a major challenge.
Climate models can help. Another session looked at whether differences in climate proxies from different regions are consistent with the physical understanding of the climate system built into a climate model. At the moment, there can be quite large difference between what some proxies show and what the models can explain – and work is ongoing to resolve these differences.
Rapid progress
Predicting global temperature in the future depends partly on how emissions change, and what policy measures are taken reduce them. But it’s also down to knowing how and why earth’s climate has changed in the past and how climate feedbacks have played a role. And it’s clear from the conference that advances in chemical analysis and modelling techniques are key to the rapid progress of the field in recent times.