Factors Contributing to Rising Global Temperatures
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Written by Mark Jeschke, Ph.D., Agronomy Manager
Key Points
- Multiple, independent datasets show that global average surface temperature has risen by about 1.8 ºF or 1.0 ºC since the late 19th Century.
- Many different factors can influence global temperature; however, the overwhelming scientific consensus is that recent warming is predominantly due to human activity.
- Adaptation of crop production systems will be necessary to ensure resiliency and sustained productivity under changing climatic conditions driven by higher temperatures.
Introduction
One of the most important factors influencing climatic trends around the world right now is rising global temperatures. Climate scientists have identified several shifts in climate trends associated with rising temperatures that will affect agricultural production, many of which are already having an impact.
This Field Facts will discuss how global temperature is measured, how scientists know that the rise in global temperature is being driven by human activity, and what that means for crop production going forward.
How do Scientists Measure Global Temperature?
Global average temperature is easy to conceptualize, but much more difficult to measure due to the variation of temperature over space and time. To get a comprehensive picture of global temperature, scientists combine thousands of individual measurements taken over land and ocean all around the world. Each individual measurement is compared to the long-term average temperature for its place and time to determine the temperature anomaly, or deviation from normal (Pidcock, 2015). The entire planet’s surface is then divided out into a grid and the average temperature anomaly for each grid square is determined (Figure 1). All daily temperature anomalies across all grid squares over the course of a year are then used to determine the annual global temperature anomaly.
Further complicating the process is the fact that temperature instrumentation and observation practices are continually changing. Historical temperature records must account for changes in measurement practices, changes in measurement locations, and changes in land use around weather stations. Temperature records must also account for spatial gaps in temperature measurements.
Figure 1. Global surface temperature anomalies on a 5 x 5 grid for July 2020 (NOAA NCEI 2021).
There are four major global surface temperature datasets scientists use. These datasets differ in the data they use, the timescales they cover and the statistical methods they employ; consequently, they do not match each other exactly. However, all four datasets show a very similar trend, which is an increase in global average surface temperature of about 1.8 ºF or 1.0 ºC since the late 19th Century (Figure 2).
Global Surface Temperature Datasets
- MLOST: Produced by the U.S. National Oceanic and Atmospheric Administration (NOAA)
- GISTEMP: Produced by the U.S. NASA Goddard Institute for Space Sciences (GISS)
- HadCRUT: Produced jointly by the UK Met Office Hadley Centre and the University of East Anglia’s Climatic Research Unit
- JMA: Produced by the Japan Meteorological Agency
The question of whether the planet is warming is not the subject of any serious scientific dispute. Multiple, independent datasets from scientific agencies around the world all show a similar trend of rising global temperatures since the late 19th Century. The next step is to understand why it is warming.
Figure 2. Global average temperature anomaly from 1880 to 2012, compared to the 1951-1980 long term average. Source: NASA Earth Observatory. View a larger image.
What is Causing the Rise in Global Temperature?
The overwhelming scientific consensus is that warming over the past century is predominantly due to human activity (Santer et al., 2019); however, there are a number of factors – both natural and human-caused – that can and do influence Earth’s temperature.
Natural Factors
Among the natural factors are long-term cycles in Earth’s orbital patterns, known as Milankovitch cycles. These are slight variations in Earth’s orbit and tilt that cause the planet to cycle between ice ages and interglacial periods (Buis, 2020).
These cycles have a large effect on Earth’s climate, but only over long periods of time. The most recent glacial period reached its maximum around 20,000 years ago with a global average temperature that was about 11 °F (6 °C) cooler than today (Tierney et al., 2020). The subsequent warming period peaked 6000-8000 years ago (Renssen et al., 2012). Since then, the effect of Earth’s orbital patterns has been a very slow, steady rate of cooling.
Figure 3. Annual global land and ocean temperature anomaly (GISTEMP 3.1) and total solar irradiance (SATIRE-T2 + PMOD), 1880-2017 (NASA, 2019).
Variations in solar activity can also affect temperature. Solar output doesn’t stay completely constant over time, with total solar irradiance varying over roughly 11-year cycles (Figure 3). However, solar output only varies by 0.15% or less over the course of these cycles so the impact on Earth temperature is minimal, only around +/-0.1 °C.
Ocean temperature cycles can cause short-term variations in climate due to changes in the balance of heat energy between the oceans and the atmosphere. The El Niño Southern Oscillation (ENSO) is an example familiar to most farmers in North America due to its potential to affect growing conditions.
Volcanic eruptions can cause a short-term cooling effect. When a volcano erupts, it can eject large quantities of sulfur dioxide, which combines with water in the stratosphere to form sulfate aerosols. These particles reflect incoming solar radiation back out into space, reducing solar transmission through the atmosphere. If the eruption is large enough, this can have a temporary global cooling effect. A relatively recent example of an eruption causing such an effect was the eruption of Mt. Pinatubo in the early 1990s. Volcanic activity can also release carbon dioxide and methane, which are both greenhouse gases, potentially leading to a warming effect.
And finally, we know that changes in atmospheric composition influence temperature. Ice core samples and other paleoclimatology records show that the concentration of greenhouse gases – carbon dioxide, specifically – has varied greatly over the history of the planet, which has been associated with large variations in global temperature.
Human Activity
Human activity can also influence temperature in ways that are analogous to some natural factors.
Industrial pollution that releases sulfur dioxide into the atmosphere contributes to stratospheric sulfate aerosols much like a volcanic eruption, reflecting solar radiation and creating a cooling effect. Global sulfur dioxide emissions have declined since the 1970s, largely due to sharp reductions in North America and Europe resulting from clean air regulations.
Greenhouse gases produced through human activities include carbon dioxide, methane, nitrous oxide and fluorinated gases. Carbon dioxide is by far the most important of these gases due to the massive quantities of it injected into the atmosphere through the burning of coal, gas, and oil. Unlike sulfur dioxide emissions, output of carbon dioxide has continued to increase. Consequently, atmospheric carbon dioxide levels have increased from around 280 ppm prior to the industrial era, to over 400 ppm today.
Modeling the Effects on Global Temperature
Computer models of Earth’s climate system allow scientists to explore the impact of each of these factors and compare their predicted effects to observed changes in temperature.
Figure 4a shows the predicted effects of natural factors, including solar output, orbital cycles, and volcanic activity. The combined predicted effect of all natural factors on temperature is relatively flat over this time period, with intermittent downward spikes associated with major volcanic events. The combined trend line does not match observed temperatures well, particularly from 1960 to present, indicating that rising temperatures are not due to natural factors.
Figure 4b shows the predicted effects of human factors, including greenhouse gases, aerosol particles, changes in land use, and changes in ozone levels. Predicted effects of changes in land use and ozone levels on temperature are relatively small. A cooling effect is associated with aerosols produced by human activity and a strong warming effect is associated with greenhouse gas emissions. The combined trend line for human factors matches observed temperatures much more closely than natural factors.
Figure 4c shows the combined effects of all natural and human factors. The combined trend line matches observed temperatures very well, suggesting that the climate model is doing a good job of accounting for the effects of the different factors.
Out of all of the factors modeled in Figure 4, greenhouse gas emissions is the only factor predicted to cause a strong warming effect and is the predominant factor to which the increase in global average temperature over the past century can be attributed.
What Does This Mean for Crop Production?
Understanding the reason for rising global temperatures is a critical prerequisite for considering its implications for crop production. Shifts in weather patterns that we have experienced in recent years cannot be dismissed as the result of a random oscillation of the planet’s climate system that will inevitably revert back to normal. We know that these changes are not random or unpredictable, but rather they are being driven by a persistent imbalance that has been introduced into Earth’s climate system through human activity. Furthermore, we know that – absent an immediate global effort to dramatically reduce greenhouse gas emissions – this imbalance will continue to grow, and its associated climatological effects will continue to intensify.
Year-to-year variation in weather will continue to exist, and some years will be hotter or wetter than others. However, we know that certain changes in climate associated with rising global temperatures that impact agriculture, such as higher night temperatures during the growing season and more intense rainfall events, will occur with greater frequency in the coming years. Because we know these changes are coming, we have the ability to start planning and implementing adaptation measures to build more resilient crop production systems.
Figure 4. Human and natural influences on global temperature, 1880-2018 (Hayhoe, et al., 2018).
References
- Buis, A. 2020. Why Milankovitch (orbital) cycles can't explain Earth's current warming. NASA Jet Propulsion Laboratory. Ask NASA Climate Blog.
- Hayhoe, K., D.J. Wuebbles, D.R. Easterling, D.W. Fahey, S. Doherty, J. Kossin, W. Sweet, R. Vose, and M. Wehner, 2018: Our Changing Climate. In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 72–144. doi: 10.7930/NCA4.2018.CH2.
- NASA. 2019. What is the sun's role in climate change? Ask NASA Climate Blog.
- NOAA National Centers for Environmental Information. 2021. Climate at a Glance: Global Mapping, published December 2020, retrieved on January 5, 2021.
- Pidcock, R. 2015. Explainer: How do scientists measure global temperature? Carbon Brief.
- Renssen, H., H. Seppa, X. Crosta, H. Goosse, and D.M. Roche. 2012. Global characterization of the Holocene Thermal Maximum. Quaternary Science Reviews 48:7-19.
- Santer, B.D., C.J.W. Bonfils, Q. Fu, J.C. Fyfe, G.C. Hegerl, C. Mears, J.F. Painter, S. Po-Chedley, F.J. Wentz, M.D. Zelinka, and C. Zou. 2019. Celebrating the anniversary of three key events in climate change science. Nature Climate Change. 9:180-182.
- Tierney, J.E., J. Zhu, J. King, S.B. Malevich, G.J. Hakim, and C.J. Poulsen. 2020. Glacial cooling and climate sensitivity revisited. Nature 584:569-573.
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