Drought in Texas
Drought in Texas

Executive summary – climate change

This chapter builds on the IPCC Fifth Assessment Report (AR5).

There is a blog covering the report.

The context is strengthening the global response to the threat of climate change, sustainable development and efforts to eradicate poverty.

It frames the knowledge-base and assessment approaches used to understand the impacts of 1.5°C global warming above pre-industrial levels, and related global greenhouse gas emission pathways.

Global warming is defined as an increase in combined surface air and sea surface temperatures averaged over the globe and a 30-year period.

  • Human-induced global warming reached approximately 1°C (±0.2°C likely range) above pre-industrial levels (the period 1850-1900) in 2017, increasing at 0.2°C (±0.1°C) per decade (high confidence).
  • Warming greater than the global average has already been experienced in many regions and seasons, with average warming over land higher than over the ocean (high confidence).
  • 20-40% of the global human population live in regions that by 2006-2015 had already experienced warming of more that 1.5ºC in at least one season.
  • Past emissions alone are unlikely to raise global-mean temperature to 1.5°C above pre-industrial levels but past emissions do commit to other changes, such as further sea level rise (high confidence).   If all human-produced emissions were reduced to zero immediately any further warming would likely be reduced to less than 0.5ºC over the next century. The level of future emissions will determine if temperature increases of more that 1.5ºC occur.
  • 1.5ºC-consistent pathways provide a one-in-two to two-in-three chance of warming remaining below 1.5ºC. Such pathways always involve limiting the cumulative emissions of long-lived greenhouse gases, including carbon dioxide and nitrous oxide, and reducing other climate forcers.  To do this we need to either bring net global emissions of long-lived greenhouses gases to zero, or achieve negative global emissions once the limit is exceeded.
  • This report assesses the projected impacts of a global average warming of 1.5C or higher. These impacts vary considerably if (a) warming remains below 1.5C or (b) levels of warming reduce to 1.5ºC after a substantial overshoot.
  • The impacts of warming beyond 1.5ºC  fall disproportionately on the poor and vulnerable.
  • Climate adaptation refers to steps taken to manage the impacts of global warming by reducing vulnerability and exposure to its harmful effects and exploiting any potential benefits
  • Ambitious actions to limit warming to 1.5ºC  are essential.
  • Multiple sources of information, including scientific evidence, are used to inform our understanding of the impacts of 1.5ºC warming.
  • There is no single answer to the question of whether it is feasible to limit global warming to 1.5ºC and to adapt to its consequences.

1.1 Assessing the knowledge base for a 1.5ºC warmer world

This report assesses a world in which global warming has been limited to a rise of 1.5ºC relative to pre-industrial levels.

Human influence on climate has been the dominant cause of global warming since the mid-20th century.  Global average surface temperatures increased by 0.85ºC between 1880 and 2012.  The world has shifted from the Holocene period (the last 11,700 years – the time since the end of the last ice age) into a new geological era the Anthropocene (the current era during which human activity has been the dominant influence on climate and the environment in general).

The rise in global atmospheric carbon dioxide concentrations since 2000 has been about 20 ppm/decade. This is up to 10 times faster than during the previous 800,000 years. These changes far exceed rates of change driven by geophysical or biosphere forces.

This temperature rise has already resulted in profound changes to human and natural systems. Many regions have experienced regional-scale warming, involving 20-40% of the global human population. There have been increases in some types of extreme weather such as droughts, floods, sea level rise and biodiversity loss.The burning of fossil fuels is a major contributor to rising greenhouse gas emissions.

The most affected people live in low and middle income countries.  Small islands, megacities, coastal regions and high mountain ranges are the most affected, along with warm-water tropical reefs and Arctic ecosystems.  The world population continues to rise, especially in high risk areas.

1.1.1 Equity and a 1.5ºC warmer world

Equity – the quality of being fair and impartial – in this context has three dimensions:

  • Intergenerational – fairness between generations
  • International – fairness between states
  • National –  fairness between individuals

This fairness involves participation in decision making and how the costs and benefits of climate change are distributed.

There are currently four inequalities in relation to the 1.5ºC warmer world:

  • Those who have historically benefitted the most from industrialization have also contributed most to the current problem and bear the greater responsibility
  • The worst impacts of climate change fall on those least responsible for the problem
  • Those most affected by climate change are not always well -represented when decisions are made regarding shaping solutions
  • Some states, groups and places are at risk of being left behind as the world moves to a low-carbon economy

1.1.2 Eradication of poverty

The United Nations Development Programme (UNDP) estimates that about 1.5 billion people globally live in poverty (in terms of needs, patterns of deprivation, or limited resources). These are especially in rural areas of South Asia and Sub-Saharan Africa. A further billion are at risk of falling into poverty.

Climate variability and climate change are recognised to be factors in exacerbating poverty, especially in parts of the world where poverty is high.  For example, agriculture is highly susceptible to temperature increases and variation in rainfall levels.  Even modest changes in these parameters can push marginalized people into poverty as they lack the means to recover from these changes.

Extreme events such as floods, droughts and heat waves can rapidly erode poor people’s assets and undermine their livelihoods.

1.1.3 Sustainable development in a 1.5ºC warmer world

This report covers a number of pathways to limiting global warming, some sustainable and some not.  Any limitation will involve substantial societal and technological changes.

The feasibility of committing to a maximum 1.5ºC pathway requires nations committing themselves to reduce their greenhouse gas emissions. The current nationally determined contributions (NDCs) only extend to 2030 and do not limit warming to 1.5ºC but track towards a warming of 3-4ºC above pre-industrial levels by 2100. This report identifies mechanisms by which economic growth can be decoupled from greenhouse gas emissions.


The appendix to this Chapter at http://report.ipcc.ch/sr15/pdf/sr15_chapter1_annex.pdf contains a number of figures to support the above text along with a detailed analysis.