Global Surface Temperature: A New Insight

This paper belongs to our Special Issue “Application of Climate Data in Hydrologic Models” [...]

The Assessment of Global Surface Temperature Change from 1850s: The C-LSAT2.0 Ensemble and the CMST-Interim Datasets

Based on C-LSAT2.0, using high- and low-frequency components reconstruction methods, combined with observation constraint masking, a reconstructed C-LSAT2.0 with 756 ensemble members from the 1850s to 2018 has been developed. These ensemble versions have been merged with the ERSSTv5 ensemble dataset, and an upgraded version of the CMST-Interim dataset with 5° × 5° resolution has been developed. The CMST-Interim dataset has significantly improved the coverage rate of global surface temperature data. After reconstruction, the data coverage before 1950 increased from 78%–81% of the original CMST to 81%–89%. The total coverage after 1955 reached about 93%, including more than 98% in the Northern Hemisphere and 81%–89% in the Southern Hemisphere. Through the reconstruction ensemble experiments with different parameters, a good basis is provided for more systematic uncertainty assessment of C-LSAT2.0 and CMST-Interim. In comparison with the original CMST, the global mean surface temperatures are estimated to be cooler in the second half of 19th century and warmer during the 21st century, which shows that the global warming trend is further amplified. The global warming trends are updated from 0.085 ± 0.004°C (10 yr)−1 and 0.128 ± 0.006°C (10 yr)−1 to 0.089 ± 0.004°C (10 yr)−1 and 0.137 ± 0.007°C (10 yr)−1, respectively, since the start and the second half of 20th century.

Reconstruction of the Interannual to Millennial Scale Patterns of the Global Surface Temperature

Climate changes are due to anthropogenic factors, volcano eruptions and the natural variability of the Earth’s system. Herein the natural variability of the global surface temperature is modeled using a set of harmonics spanning from the inter-annual to the millennial scales. The model is supported by the following considerations: (1) power spectrum evaluations show 11 spectral peaks (from the sub-decadal to the multi-decadal scales) above the 99% confidence level of the known temperature uncertainty; (2) spectral coherence analysis between the independent global surface temperature periods 1861–1937 and 1937–2013 highlights at least eight common frequencies between 2- and 20-year periods; (3) paleoclimatic temperature reconstructions during the Holocene present secular to millennial oscillations. The millennial oscillation was responsible for the cooling observed from the Medieval Warm Period (900–1400) to the Little Ice Age (1400–1800) and, on average, could have caused about 50% of the warming observed since 1850. The finding implies an equilibrium climate sensitivity of 1.0–2.3 °C for CO2 doubling likely centered around 1.5 °C. This low sensitivity to radiative forcing agrees with the conclusions of recent studies. Semi-empirical models since 1000 A.D. are developed using 13 identified harmonics (representing the natural variability of the climate system) and a climatic function derived from the Coupled Model Intercomparison Project 5 (CMIP5) model ensemble mean simulation (representing the mean greenhouse gas—GHG, aerosol, and volcano temperature contributions) scaled under the assumption of an equilibrium climate sensitivity of 1.5 °C. The harmonic model is evaluated using temperature data from 1850 to 2013 to test its ability to predict the major temperature patterns observed in the record from 2014 to 2020. In the short, medium, and long time scales the semi-empirical models predict: (1) temperature maxima in 2015–2016 and 2020, which is confirmed by the 2014–2020 global temperature record; (2) a relatively steady global temperature from 2000 to 2030–2040; (3) a 2000–2100 mean projected global warming of about 1 °C. The semi-empirical model reconstructs accurately the historical surface temperature record since 1850 and hindcasts mean surface temperature proxy reconstructions since the medieval period better than the model simulation that is unable to simulate the Medieval Warm Period.

Global surface temperature response to 11-year solar cycle forcing consistent with general circulation model results

The 11-year solar cycle is associated with a roughly 1Wm-2 trough-to-peak variation in total solar irradiance and is expected to produce a global temperature response. The sensitivity of this response is, however, contentious. Empirical best estimates of global surface temperature sensitivity to solar forcing range from 0.08 to 0.18 K [W m-2 ]-1. In comparison, best estimates from general circulation models forced by solar variability range between 0.03-0.07 K [W m-2]-1, prompting speculation that physical mechanisms not included in general circulation models may amplify responses to solar variability. Using a lagged multiple linear regression method, we find a sensitivity of globalaverage surface temperature ranging between 0.02-0.09 K [W m-2]-1, depending on which predictor and temperature datasets are used. On the basis of likelihood maximization, we give a best estimate of the sensitivity to solar variability of 0.05 K [W m-2]-1 (0.03-0.09 K, 95% c.i.). Furthermore, through updating a widely-used compositing approach to incorporate recent observations, we revise prior global temperature sensitivity best estimates of 0.12 to 0.18 K [W m-2]-1 downwards to 0.07 to 0.10 K [W m-2]-1. The finding of a most-likely global temperature response of 0.05 K [W m-2]-1 supports a relatively modest role for solar cycle variability in driving global surface temperature variations over the 20th century and removes the need to invoke processes that amplify the response relative to that exhibited in general circulation models.

Everything, almost everywhere, is slowing down

In the decades, and especially the years, immediately before the 2020 pandemic swept the world, almost everything that we routinely measured about human lives, worldwide, was already slowing down. Much was still rising, our levels of debt, the information we produced, our numbers on the planet, but it was not rising as fast as it has been rising before. Those few things that were still accelerating worldwide before the pandemic: air flights, pollution, global surface temperature—all slowed with the onset of the pandemic. We cannot know what will happen next, but we should not accept suggestions of renewed acceleration unless they are accompanied with new and convincing evidence.

Climate Change and European Cities

The year 1950 has been a tipping point for Europe, as most of the European population became more urban than rural. Since that moment such a transition never stopped, and, projections say that by 2050, the number of urban inhabitants will approximately reach 75% of the total population in Europe, likely imposing further urban sprawl in one of the already most urbanized regions worldwide. As cities are responsible for 75% of the global carbon dioxide emissions, a questionabout how cities are dealing with climate change raises. Climate change threatens cities in numerous ways and at different scales. For instance, urbanization entails local increase in urban temperature, compared to the rural environs, known as Urban HeatIsland (UHI) effect. Both big and small-sized European cities are experiencing UHI. Previous research shows that in Paris, Rome and Barcelona, the UHI is as high as 8, 5 and 8.2 °C, respectively. In addition to urban and microscale temperature surges, anthropogenicclimate change has amplifiedthe intensity and frequency of mesoscale warming phenomena: heat waves. Particularly relevant have been the heat waves recorded in 2003, 2006, 2007, 2010, 2014, 2015 and 2017. In Europe, from June to August 2003, the heat wave caused about 35000 deaths. In 2018, persistent high temperature anomalies were recorded in Europe, and in particular in Scandinavia and Northern Europe. Most important, estimates show that mesoscale warming phenomena will become more frequent in the coming years. On top of these warming phenomena, global land-ocean temperatures are continuing increasing in the last decades. In 2017 the global surface temperature resulted being 0.9 °C higher than the average global surface temperature relative to 1951-1980. The increase in global temperature entails the ice cap melting which causes sea level rise. At present, globally, sea level is 89.7 mm (±0.80 mm) higher than in 1993. In particular, in Europe, both northern European countries and Mediterraneanones, have experienced, in the last 45 years a sea level rise ranging from 0.5 to 3 and from 0.5 to 4 mmper year, respectively. Projections show that, in the coming years, both Northern and Southern European countries will be affected by an increase in the sea level ranging from 0.1 to >0.4 m. As sea level is projected to rise in the coming years, coastal cities—which represent 90% of urban areas globally—will likely be threatened by flooding. Without adaptation strategies, the number of people in Europe annually affected by coastal flooding will be about 0.05 -0.13% of the 27 EU population in 2010. In particular, the Netherlands is ranked among the 20 most exposed countries worldwideto flooding, with potential economic loss of approximately US $1670 billion. Although climate change is a well-known phenomenon—already in 1988 Dr. James Hansen predicted that the increase in greenhouse gases would have led in 2017 to an increase in global temperature of about 1.03 °C compared to the average temperature recorded from 1950 until1980—the global greenhouse gas emissions continue rising, showing that climate negotiations are either still gridlocked or not sufficient to decrease climate altering emissions. If, on the one handinternational negotiations are slow,on the other hand, cities, especially in the last years, are proactivelyimplementingadaptationand mitigation plans. 66% of the European cities have adopted adaptation or mitigation plans. In the list of the top 5 countries with the highest percentage of cities with mitigation or adaptation plans there are Poland, Germany, Ireland, Finland, and Sweden. However, such plans are compulsory just in a minority of countries (i.e., Denmark, France, Slovakia and the UK). As international climate change negotiations fail in addressing climate urgency, as demonstrated by COP24 held in Katowice (Poland) on December 2018, cities, which are among the major causes and the main victims of climate change, have demonstratedto own the right political agility to put in place efficient mitigation and adaptation urban plans. However, as isolated actions would not lead to any measurable global effect, just coordinated efforts, harmonized either at upper scales or among municipalities globally, can provide global mitigation benefits.