19th century weather data is helping climate scientists predict the future


Linden Ashcroft, Universitat Rovira i Virgili; Howard Bridgman, University of Newcastle, and Ken Thornton, University of Newcastle

The 19th-century English historian Lord Acton famously advised people to live in both the future and the past, and said “those who do not live in the past cannot live in the future”.

It may seem a stretch to apply this famous quotation to climate research, but we can’t possibly understand how the climate will change in the future without first understanding how it changed in the past.

Australia’s official climate record, kept by the Bureau of Meteorology, begins in 1910. But historical climate records kept before the development of national meteorological organisations are valuable tools for improving our understanding of what has happened in the past.

They also put the present into a long-term context, and improve climate models used to predict the future.

What can old numbers really tell us?

One thing historical records can help us understand is extreme weather events – the aspect of climate change that has people most concerned. How can we prepare cities and buildings for storms in the future without understanding what previous storms have done?

It is true that historical observations have reliability issues and are sometimes hard to compare to modern observations, which are recorded in a standard way. However, old weather records can still tell us a lot about year-to-year changes, and there are many ways to minimise reliability problems.

There are several climate and weather analysis products that recreate how the atmosphere behaved over time. In the Southern Hemisphere, these climate tools are generally uncertain until the mid-20th century, due to a lack of – you guessed it — long-term data.

Historical records can also help us hone climate models for predicting the future. Some of the atmospheric and oceanic features that dominate our climate have cycles that can last several decades. This means that modern climate data starting in the 20th century may only capture one or two cycles of a particular feature, making it hard to train climate models on the full range of our climate variability.

Historical weather records are also important for past climate analysis. Extracting the climate signal from tree rings, ice cores, or documentary data, requires instrumental climate records for comparison. The longer the climate records are, the better this comparison will be.

What exists for Australia?

In the past few years, concerted efforts at the Bureau of Meteorology and several universities have been able to recover and analyse a large amount of historical climate data for Australia. Most of these observations come from Australia’s southeast, as this is the region that was first colonised by Europeans.

There are now studies that explore temperature, rainfall and atmospheric pressure variability in southeastern Australia back to the 1860s. Several studies have even rescued data from 1788.

With these newly recovered observations, we have learned a lot about Australia’s climate in the 19th century, as well as the early years of English colonisation. But there is still a lot we don’t know.

In particular, the majority of our old weather data come from coastal locations, where the weather is more sensitive to local factors rather than large-scale features such as the El Niño–Southern Oscillation (ENSO).

A rare opportunity

In 2011, some weather diaries were donated to the University of Newcastle and University of New England. The diaries contain 45 years of daily handwritten weather observations of a Mr Algernon Belfield taken on his 8,000-hectare property, Eversleigh, near Armidale in northern New South Wales.

A H Belfield at Eversleigh
Belfield Family

A pastoralist, amateur meteorologist and astronomer, Belfield took these meticulous weather observations every morning at 9am from June 1877 until August 1922, a month before his death.

His observations continued through the period of the 1891 shearers’ strike, the Boer War, Australia’s Federation, the First World War and the Centennial and Federation droughts.

Belfield’s diaries also span the period that inspired Dorothea Mackellar’s famous ode to Australia, My Country.

The last few decades of the 19th century were indeed times of “droughts and flooding rains”, thanks to a string of La Niña and El Niño phases of ENSO.

Belfield weather diaries
Ken Thornton (Author)

Belfield’s steady hand captured the weather at Eversleigh during a time of dramatic variability before the impact of human-induced climate change, in a region where the climate is highly correlated with ENSO.

His detailed records, therefore, provide us with a unique opportunity to uncover more about this period in our climate history than ever before.

The handwritten records are scanned but need to be transcribed into a digitised format. We are looking for volunteers to help us with this important task of recovering our climate history. If you are interested, please contact us here, to help shed light on Australia’s past, present and future climate.

The Conversation

Linden Ashcroft, Senior Researcher, Universitat Rovira i Virgili; Howard Bridgman, Conjoint Professor, University of Newcastle, and Ken Thornton, Affiliate, Cultural Collections, University of Newcastle

This article was originally published on The Conversation. Read the original article.

Burning fossil fuels is responsible for most sea-level rise since 1970


Aimée Slangen, Utrecht University and John Church, CSIRO

Global average sea level has risen by about 17 cm between 1900 and 2005. This is a much faster rate than in the previous 3,000 years.

The sea level changes for several reasons, including rising temperatures as fossil fuel burning increases the amount of greenhouse gases in the atmosphere. In a warming climate, the seas are expected to rise at faster rates, increasing the risk of flooding along our coasts. But until now we didn’t know what fraction of the rise was the result of human activities.

In research published in Nature Climate Change, we show for the first time that the burning of fossil fuels is responsible for the majority of sea level rise since the late 20th century.

As the amount of greenhouse gases we are putting into the atmosphere continues to increase, we need to understand how sea level responds. This knowledge can be used to help predict future sea level changes.


CSIRO

Measuring sea level

Nowadays, we can measure the sea surface height using satellites, so we have an accurate idea of how the sea level is changing, both regionally and in the global mean.

Prior to this (before 1993), sea level was measured by tide gauges, which are spread unevenly across the world. As a result, we have a poorer knowledge of how sea level has changed in the past, particularly before 1960 when there were fewer gauges.

Nevertheless, the tide gauge measurements indicate that global mean sea level has increased by about 17 cm between 1900 and 2005.

What drives sea level rise?

The two largest contributors to rising seas are the expansion of the oceans as temperatures rise, loss of mass from glaciers and ice sheets, and other sources of water on land. Although we now know what the most important contributions to sea-level rise are, we did not know what is driving these changes.

Changes in sea level are driven by natural factors such as natural climate variability (for example El Niño), ongoing response to past climate change (regional warming after the Little Ice Age), volcanic eruptions, and changes in the sun’s activity.

Volcanic eruptions and changes in the sun affect sea level across years to decades. Large volcanic eruptions can cause a temporary sea-level fall because the volcanic ash reduces the amount of solar radiation reaching the ocean, thus cooling the ocean.

Humans have also contributed to sea level rise by burning fossil fuels and increasing the concentration of greenhouse gases in the atmosphere.

Separating the causes

We used climate models to estimate ocean expansion and loss of mass from glaciers and ice sheets for each of the individual factors responsible for sea level change (human and natural). To this we added best estimates of all other known contributions to sea level change, such as groundwater extraction and additional ice sheet contributions.

We then compared these model results to the observed global mean 20th century sea-level change to figure out which factor was responsible for a particular amount of sea level change.

Over the 20th century as a whole, the impact of natural influences is small and explains very little of the observed sea-level trend.

The delayed response of the glaciers and ice sheets to the warmer temperatures after the Little Ice Age (1300-1870 AD) caused a sea-level rise in the early 20th century. This explains much of the observed sea-level change before 1950 (almost 70%), but very little after 1970 (less than 10%).

The human factor

The largest contributions to sea-level rise after 1970 are from ocean thermal expansion and the loss of mass from glaciers in response to the warming from increasing greenhouse gas concentrations. This rise is partly offset by the impact of aerosols, which on their own would cause a cooling of the ocean and less melting of glaciers.

The combined influence of these two factors (greenhouse gases and aerosols) is small in the beginning of the century, explaining only about 15% of the observed rise. However, after 1970, we find that the majority of the observed sea-level rise is a direct response to human influence (nearly 70%), with a slightly increasing percentage up to the present day.

When all factors are considered, the models explain about three quarters of the observed rise since 1900 and almost all of the rise over recent decades (almost 90% since 1970).

The reason for this difference can be found either in the models or in the observations. The models could underestimate the observed rise before 1970 due to, for instance, an underestimated ice sheet contribution. However, the quality and number of sea level observations before the satellite altimeter record is also less.

Tipping the scales

Our paper shows that the driving factors of sea-level change have shifted over the course of the 20th century.

Past natural variations in climate were the dominant factor at the start of the century, as a result of glaciers and ice sheets taking decades to centuries to adapt to climate change.

In contrast, by the end of the 20th century, human influence has become the dominant driving factor for sea-level rise. This will probably continue until greenhouse gas emissions are reduced and ocean temperatures, glaciers and ice sheets are in equilibrium with climate again.


John will be on hand for an Author Q&A between 4 and 5pm AEST on Tuesday, April 12, 2016. Post your questions in the comments section below.

The Conversation

Aimée Slangen, Postdoctoral research fellow, Institute for Marine and Atmospheric Research, Utrecht University and John Church, CSIRO Fellow, CSIRO

This article was originally published on The Conversation. Read the original article.