insight

By Dr. Norm Catto

How much of ongoing climate change is due to people? How do we know that we contribute at all?
Current discussion about climate change is based on differences of scientific opinion concerning both the significance and magnitude of the changes recorded in temperature and precipitation statistics, as well as on the “global” nature of the change, and the appropriateness of applying records from one region to another. One must always remember that the discussion of climate deals with long-term averages, over several decades. The weather for each individual day, month, season, and year will continue to vary within that overall pattern.

In the scientific community, the discussion centres around whether human activity is responsible for all of the climate change in the past 150 years, or only for part of it, or if it is dominantly natural in origin. Scientists agree that climate change is happening, as it has since the origin of Earth 4.6 billion years ago, but we’re not all in agreement as to exactly how much blame should be put on our species, and how much of the change is natural.

The suggestion that humans are responsible for at least a significant component of climate change comes from several lines of evidence. Theoretical considerations suggest that discharging carbon dioxide and methane into the atmosphere will cause atmospheric warming, as can be directly observed above all cities in the developed world where energy use involves burning fossil fuels. Measurements of carbon dioxide and methane preserved in ice cores from Antarctica and Kallaallit Nunaat indicate that the concentrations have been relatively stable over the past 10,000 years. However, since 1800, the concentration of carbon dioxide in the ice and the atmosphere has increased by about 30 per cent, and the concentration of methane has doubled.

We’re not all in agreement as to exactly how much blame should be put on our species, and how much of the change is natural.

Increases in temperature have also been recorded in most areas of North America since 1845. In the Rocky Mountains, glacial recession and tree ring records (dendrochronology) indicate that glaciers today occupy less area than they did 9,000 years ago. Most of these glaciers have retreated more within the past 150 years than they had advanced in the previous 9,000 years. This indicates that climate warming and drying have occurred in this region, and at an accelerated rate since 1845. Similar conclusions have been drawn from studying lakes in the Canadian Prairies. In Canada as a whole, Environment Canada statistics indicate that six of the eight warmest years on record (i.e. since Fahrenheit invented the thermometer, around 1800) have occurred since 1992.

Unfortunately, these statistics are limited in their usefulness by the relatively short time that they represent. The longest accurate temperature records, from Europe and New England, span the period since Fahrenheit’s thermometer was considered to be sufficiently reliable – less than 200 years. In most areas of western North America, accurate temperature records encompass 150 years at most; in arctic Canada, reliable observations date from the Second World War. Climate, by definition, involves long term averages. If climate change is to be recognized, it is necessary to look beyond the numerical records.

Climate changes in the historic past are assessed by studying human records based on climate-dependent human activities. Among those studied are the dates of annual grape harvests from European Monasteries; the timing of cherry blossoms at Shinto temples in Japan; the success or failure of wheat cultivation in the North Atlantic regions; iceberg numbers and time of ships’ encounters off Iceland’s coast; and trapping data from the Hudson’s Bay Company. All of these human activities can be used as analogies to calculate the climate at those times.

For earlier periods, or in areas where there are no written records available, proxy data from natural systems are used. Proxy data are those that can be used to infer climate. Among the styles of proxy data used to assess past climates from 4.6 billion years ago to 1800 AD are ice core records from continental and mountain glaciers; pollen analysis from lake sediments; other microfauna and flora, including diatoms, insect remains, and marine plankton; dendrochronology; glacial advances and retreats; changes in ranges of animal and plant species; changes in soil development and type; and changes in the types of sediments or landforms, such as glacial features and sand dunes.

In the countries surrounding the North Atlantic Ocean, abundant proxy data exists for the last 1,000 years. This recorded data, along with the impressions of contemporary writers, allows reconstruction of the climate patterns. The results show that much regional variability existed, even over very short distances: adjacent mountain valleys record different responses to the same weather and climate events. This should not be surprising, as modern weather events produce equally diverse responses and effects. When results are compared across the expense of western Europe and mid-latitude North America, however, a pattern of consistent climate change emerges.

In general, the period from about 700 to 1300 A.D. was marked by relatively warm conditions (generally, slightly less warm than those at the end of the 20th century). This interval is referred to as the Little Climatic Optimum or the Mediaeval Warming. Following this, temperatures cooled, resulting in glacial advances in alpine areas. This event, referred to as the Neoglacial or Little Ice Age, persisted until the mid-19th century. The Neoglacial was followed by a cycle of climate warming, which is currently in progress.

Proxy data, historical records, and numerical temperature and precipitation observations allow comparison between what has happened under the purely natural circumstances that undoubtedly existed prior to 1800 AD, and what has happened since then. The most striking difference is not in the type of climate changes, or the areas of occurrence, or the consequences to organisms: it is the speed at which the changes are occurring. Changes that required hundreds, thousands, or tens of thousands of years in the natural and geological records are now seen within the span of decades. The rates of climate change are increasing, along with increased human production of carbon dioxide and methane. The acceleration of the rate of change began in the early 1800s, just as human consumption of fossil fuels increased. Ongoing climate changes directly above cities are proportionate to the amount of energy consumed by each, with differences evident due to city size, lifestyle, and economic wealth. Taken all together, the acceleration of climate change cannot be explained solely by natural causes: human activity is the only factor that has changed substantially in Earth’s climate system since 1800.

So, we are not responsible for all climate change: natural factors continue to operate, as they have for 4.6 billion years. However, the accelerated rate indicates that humans have made a contribution in the past 200 years.

Regardless of the cause, however, impacts of climate change are happening today, and we will all have to adapt. Considering our resilience in the face of the province’s traditional weather, we should be able to cope with that in the future.

Dr. Norm Catto has been with Memorial's Department of Geography since 1989. His research and teaching interests include the impacts of climate change, and the necessary adaptations, in Newfoundland and Labrador, throughout Canada, and in Russia.