causality L3

Early Warnings & Evidence

• 1988 James Hansen (for NASA): testified to U.S. Senate about climate risks.

  • Outlined ___ (strong vs. weak mitigation).

  • His ___ → closely match observed ___.

scenarios, projections, warming

• Ice ____ Data:

  • Show ____ concentrations rose/fell with temperatures over hundreds of thousands of years.

  • Order (past): ___ change → then ____ change (lag).

  • Trigger: ____ cycles (orbital/solar shifts).

CO₂ acted as ____ amplifier, not initial driver.

core, CO2, temperature, CO2, Milankovitch, feedback

• Shift since ~1900:

  • Now CO₂ rises ___ ___ increases.

  • Huge jump from 280 ppm → 425 ppm CO₂.

  • Since 1970: +2 ppm CO₂/year.

before temperature

Greenhouse Gases

• Carbon dioxide (CO₂):

  • Lifetime: ~100 years.

  • Primary driver of long-term warming.

• Methane (CH₄):

  • Lifetime: ~10 years.

  • 20–30% of warming since ____

  • Much stronger heat-trapping capacity per molecule than CO₂.

  • Sources:

    • Natural → ____.

    • Human → agriculture (___), fossil fuels, ____.

  • Positive feedback: thawing ______ + shallow ocean deposits and ___ release methane.

wetlands, cows, permafrost, arctic

Observed Warming

• 1900 → +0.7 °C above pre-industrial.

• Since pre-industrial → +1.5 °C today.

• Half of all carbon ever burned = since ____.

• In short, scientific certainty that natural variations DON'T explain current warming

1990

Observed Warming

• 1900 → +0.7 °C above pre-industrial.

• Since pre-industrial → +1.5 °C today.

• Half of all carbon ever burned = since 1990.

• In short, scientific certainty that natural variations DON'T explain current warming 

  • Level of incoming solar energy has ____ changed (____ very slightly amid marked warming of past 4 decades) 

  • Cycles affect changes over much longer term time-frames → nature and pace of change of present warming different than past climate cycles 

    • Across pleistocene, changing atmospheric GHG concentrations ___ temperature, triggered by Milankovitch cycles 

    • Clear ____ human fingerprints on changing atmospheric chemistry 

    • Clear evidence range of key ___ feedbacks in motion

barely, decreasing, after, anthropogenic, postive

***Scientific Consensus

• 1980s–90s: growing clarity of human-driven climate change.

• 1990s: National Academies (US, Japan, Germany, UK) confirmed human causation.

• _____ (Intergovernmental Panel on Climate Change):

  • ___ Earth science, aim is _____ relevance

  • 3 Working Groups:

    1. ___.

    2. Impacts, adaptation, vulnerability.

    3. _____.

  • Over time: increasing ____ about drivers, risks, urgency of big GHG emission ___.

  • ____ refined and validated with observed data.

IPCC, assesses, policy, science, mitigation, certainty, cuts, models

• 2019 IPCC Special Report:

  • Beyond +1.5 °C → dangerous positive ____.

    1. Window closing to get atmospheric GHG concentrations to a level where this could be possible 

  • To stay under 1.5: emissions must be cut in ___ by 2030.

  • Current trajectory: +16% emissions from 2010 → 2030.

Locks in ~___ °C warming.

feedbacks, half, 2.7

Why Natural Variations Can’t Explain Current Warming

• Solar input: barely changed (slightly decreased) in last 40 years.

• Past cycles (Milankovitch) → operate over much longer timescales.

• Modern pace + order flipped: CO₂ ____ ___ rise

• Anthropogenic fingerprints on atmospheric chemistry.

leads temp

Positive Feedbacks

• Ice-Albedo Effect:

  • Ice __ 50–80% of incoming solar ___ back to space.

  • Cooling → more ice → stronger reflection.

  • Warming → less ice → more __ → amplifies warming.

reflects radiation absorption

• __ (permanent snow & ice):

  • Mainly ___ & ____*****.

  • Ice shelves = land-attached but floating over sea.

  • Increasing ____ → ___ melt.

  • ___: more days of surface melt each year.

cryosphere, greenland, antarctica, instability, accelerating, greenland

• Sea ____ (Arctic + Antarctic):

  • Expands in winter, shrinks in summer (_____ effect).

  • Summer minimum around September.

  • North Pole warming > global average.

  • Arctic sea ice: halved in area + thickness since 1980s.

  • Arctic ice pack = now ¼ of past volume.

  • Freeze-up occurring ___ in the year.

ice, accordian, later

Review Points (Test-Focused)

• Milankovitch Cycles (3 orbital changes):

  • Alter solar energy distribution → trigger ice ___/interglacials.

Important: trigger is solar ____ changes***

ages, radiation